Anthony-Claret is a software Engineer, entrepreneur and the founder of Codewit INC. Mr. Claret publishes and manages the content on Codewit Word News website and associated websites. He's a writer, IT Expert, great administrator, technology enthusiast, social media lover and all around digital guy.
Guinea-Bissau’s parliament has elected the speaker as the West African state’s interim leader, a day after the president was assassinated. Raimundo Pereira must organise a fresh presidential election within 60 days.
Anthony-Claret is a software Engineer, entrepreneur and the founder of Codewit INC. Mr. Claret publishes and manages the content on Codewit Word News website and associated websites. He's a writer, IT Expert, great administrator, technology enthusiast, social media lover and all around digital guy.
Three options for connecting over a telephone service:
Dial-up connections.
Integrated Services Digital Network(ISDN) – A method of sending voice and data information on a digital phone line.
Basic ISDN – Two 64Kbps B-channels with one 16Kbps D channel is provided. The D-channel is used for call control and setup. Basic ISDN can provide 128Kbps speed capability.
Primary ISDN – 23 B-channels and one D channel is provided.
Leased Lines – This involves the leasing of a permanent telephone line between two locations.
Remote Communication Protocols
Serial Line Internet Protocol (SLIP) – Allows computers to connect to the internet with a modem. No error checking or data compression is supported. Only the TCP/IP protocols are supported.
Point to Point Protocol (PPP) – Provides error checking and data compression. Also supports multiple network protocols such IPX/SPX and NetBEUI in addition to TCP/IP. Supports dynamic allocation of IP addresses.
Remote Access Service
Remote Access Service (RAS) with Windows NT allows users connecting to the network using a modem to use network resources. RAS may be called dial up networking (DUN) depending on the version of Windows you are using. The NT RAS server can handle 256 connections. Windows NT RAS servers provide the following security features:
User account security
Encryption between the DUN (dial up networking) client and the server
Callback capability
The client software is called Dial up networking (DUN) in windows NT4 and Windows95. For NT 3.51 and Windows 3.1 it is called a RAS client. These clients may be used to connect to the internet through an internet service provider (ISP). View Advance Level
Further Reading:
TCP/IP Illustrated, Volume1, The Protocols
Author:W. Richard Stevens, Publisher: Addison Wesley. ISBN 020163346
Anthony-Claret is a software Engineer, entrepreneur and the founder of Codewit INC. Mr. Claret publishes and manages the content on Codewit Word News website and associated websites. He's a writer, IT Expert, great administrator, technology enthusiast, social media lover and all around digital guy.
This section may be skipped by those more interested on the software aspects of networking or those learning networking, but all readers should at some time be aware of the terminology used in this section since they are used with regard to cabling. If this section is skipped by those learning networking, it should be read later. This section should be read by those who plan to physically install their own network. Types of Transmission
   – Baseband – Data bits are defined by discrete signal changes.
   – Broadband – Uses analog signals to divide the cable into several channels with each channel at its own frequency. Each channel can only transmit one direction.
Physical media
   – Twisted pair – Wire is twisted to minimize crosstalk interference. It may be shielded or unshielded.    – UTP-Unshielded Twisted Pair. Normally UTP contains 8 wires or 4 pair. 100 meter maximum length. 4-100 Mbps speed.
       – STP-Shielded twisted pair. 100 meter maximum length. 16-155 Mbps speed. Lower electrical interference than UTP.
   – Coaxial – Two conductors separated by insulation such as TV 75 ohm cable. Maximum length of 185 to 500 meters.
       – Thinnet – Thinnet uses a British Naval Connector (BNC) on each end. Thinnet is part of the RG-58 family of cable*. Maximum cable length is 185 meters. Transmission speed is 10Mbps. Thinnet cable should have 50 ohms impedance and its terminator has 50 ohms impedance. A T or barrel connector has no impedance.
  1.         1. Thicknet – Half inch rigid cable. Maximum cable length is 500 meters. Transmission speed is 10Mbps. Expensive and is not commonly used. (RG-11 or RG-8). A vampire tap or piercing tap is used with a transceiver attached to connect computers to the cable. 100 connections may be made. The computer has an attachment unit interface (AUI) on its network card which is a 15 pin DB-15 connector. The computer is connected to the transceiver at the cable from its AUI on its network card using a drop cable.
   Coax cable types:
  1.          * RG-58 /U – 50 ohm, with a solid copper wire core.          * RG-58 A/U* – 50 ohm, with a stranded wire core.          * RG-58 C/U* – Military version of RG-58 A/U.          * RG-59 – 75 ohm, for broadband transmission such as cable TV.          * RG-62 – 93 ohm, primarily used for ArcNet.          * RG-6 – Used for satellite cable (if you want to run a cable to a satellite!).
   *Only these are part of the IEEE specification for ethernet networks.
  1. Fiber-optic – Data is transmitted using light rather than electrons. Usually there are two fibers, one for each direction. Cable length of 2 Kilometers. Speed from 100Mbps to 2Gbps. This is the most expensive and most difficult to install, but is not subject to interference. Two types of cables are:         1. Single mode cables for use with lasers.         2. Multimode cables for use with Light Emitting Diode (LED) drivers.
Cable Standards
This section may be skipped by those more interested on the software aspects of networking or those learning networking, but all readers should at some time be aware of the terminology used in this section since they are used with regard to cabling. If this section is skipped by those learning networking, it should be read later. This section should be read by those who plan to physically install their own network. Types of Transmission
  1. Baseband – Data bits are defined by discrete signal changes.   2. Broadband – Uses analog signals to divide the cable into several channels with each channel at its own frequency. Each channel can only transmit one direction.
Physical media
  1. Twisted pair – Wire is twisted to minimize crosstalk interference. It may be shielded or unshielded.          * UTP-Unshielded Twisted Pair. Normally UTP contains 8 wires or 4 pair. 100 meter maximum length. 4-100 Mbps speed.          * STP-Shielded twisted pair. 100 meter maximum length. 16-155 Mbps speed. Lower electrical interference than UTP.   2. Coaxial – Two conductors separated by insulation such as TV 75 ohm cable. Maximum length of 185 to 500 meters.         1. Thinnet – Thinnet uses a British Naval Connector (BNC) on each end. Thinnet is part of the RG-58 family of cable*. Maximum cable length is 185 meters. Transmission speed is 10Mbps. Thinnet cable should have 50 ohms impedance and its terminator has 50 ohms impedance. A T or barrel connector has no impedance.         2. Thicknet – Half inch rigid cable. Maximum cable length is 500 meters. Transmission speed is 10Mbps. Expensive and is not commonly used. (RG-11 or RG-8). A vampire tap or piercing tap is used with a transceiver attached to connect computers to the cable. 100 connections may be made. The computer has an attachment unit interface (AUI) on its network card which is a 15 pin DB-15 connector. The computer is connected to the transceiver at the cable from its AUI on its network card using a drop cable.      Coax cable types:          * RG-58 /U – 50 ohm, with a solid copper wire core.          * RG-58 A/U* – 50 ohm, with a stranded wire core.          * RG-58 C/U* – Military version of RG-58 A/U.          * RG-59 – 75 ohm, for broadband transmission such as cable TV.          * RG-62 – 93 ohm, primarily used for ArcNet.          * RG-6 – Used for satellite cable (if you want to run a cable to a satellite!).      *Only these are part of the IEEE specification for ethernet networks.   3. Fiber-optic – Data is transmitted using light rather than electrons. Usually there are two fibers, one for each direction. Cable length of 2 Kilometers. Speed from 100Mbps to 2Gbps. This is the most expensive and most difficult to install, but is not subject to interference. Two types of cables are:         1. Single mode cables for use with lasers.         2. Multimode cables for use with Light Emitting Diode (LED) drivers.
Cable Standards
The Electronic Industries Association and Telecommunications Industries Association (EIA/TIA) defined a standard called EIA/TIA 568 which is a commercial building wiring standard for UTP cable. It defines transmission speed and twists per foot. Category    Speed    Notes 1    None    Used for old telephone systems 2    4Mps     3    10Mps    The minimum category for data networks 4    16Mps     5    100Mps    Cat 5 network cable, used by most networks today 6        Data patch, Two pair with foil and braided shield 7        Undefined 8        Flat cable for under carpets with two twisted pair 9        Plenum cable with two twisted pair. It is safe if you’re having a fire.
The maximum transmission length is 100 meters. This cable is susceptible to interference. STP
Shielded twisted pair has a maximum cable length of 100 meters (328 feet). Data rate from 16 to 155 Mbps. Cables require special connectors for grounding but this cabling method resists electrical interference and is less susceptible to eavesdropping. Costs more than UTP or Thinnet, but not as much as Thicknet or Fiber-optic. Terms
   * Attenuation – Signal loss due to impedance.    * Bandwidth – Indicates the amount of data that can be sent in a time period. Measured in Mbps which is one million bits per second.    * Impedance – The amount of resistance to the transmission device.    * Interference – Electromagnetic Interference (EMI). Crosstalk – When wires pick up electromagnetic signals from nearby wires also carrying signals.    * Plenum – Space above a false ceiling in an office area where heat ducts and cables may be run. Plenum cabling is special fire resistant cabling required for use in these areas due to fire hazards.    * Shielding – Used to minimize interference
Further Reading:
TCP/IP Illustrated, Volume1, The Protocols
Author:W. Richard Stevens, Publisher: Addison Wesley. ISBN 0201633469
Anthony-Claret is a software Engineer, entrepreneur and the founder of Codewit INC. Mr. Claret publishes and manages the content on Codewit Word News website and associated websites. He's a writer, IT Expert, great administrator, technology enthusiast, social media lover and all around digital guy.
Internet Control Message Protocol (ICMP) defined by RFC 792 and RFC 1122 is used for network error reporting and generating messages that require attention. The errors reported by ICMP are generally related to datagram processing. ICMP only reports errors involving fragment 0 of any fragmented datagrams. The IP, UDP or TCP layer will usually take action based on ICMP messages. ICMP generally belongs to the IP layer of TCP/IP but relies on IP for support at the network layer. ICMP messages are encapsulated inside IP datagrams.
ICMP will report the following network information:
Timeouts
Network congestion
Network errors such as an unreachable host or network.
The ping command is also supported by ICMP, and this can be used to debug network problems.
ICMP Messages:
The ICMP message consists of an 8 bit type, an 8 bit code, an 8 bit checksum, and contents which vary depending on code and type. The below table is a list of ICMP messages showing the type and code of the messages and their meanings.
Type
Codes
Description
Purpose
0
0
Echo reply
Query
3
0
Network Unreachable
Error
3
1
Host Unreachable
Error
3
2
Protocol Unreachable
Error
3
3
Protocol Unreachable
Error
3
4
Fragmentation needed with don’t fragment bit set
Error
3
5
Source route failed
Error
3
6
Destination network unknown
Error
3
7
Destination host unknown
Error
3
8
Source host isolated
Error
3
9
Destination network administratively prohibited
Error
3
10
Destination host administratively prohibited
Error
3
11
Network Unreachable for TOS
Error
3
12
Host Unreachable for TOS
Error
3
13
Communication administratively prohibited by filtering
Error
3
14
Host precedence violation
Error
3
15
Precedence cutoff in effect
Error
4
0
Source quench
Error
5
0
Redirect for network
Error
5
1
Redirect for host
Error
5
2
Redirect for type of service and network
Error
5
3
Redirect for type of service and host
Error
8
0
Echo request
Query
9
0
Normal router advertisement
Query
9
16
Router does not route common traffic
Query
10
0
Router Solicitation
Query
11
0
Time to live is zero during transit
Error
11
1
Time to live is zero during reassembly
Error
12
0
IP header bad
Error
12
1
Required option missing
Error
12
2
Bad length
Error
13
0
Timestamp request
Query
14
0
Timestamp reply
Query
15
0
Information request
Query
16
0
Information reply
Query
17
0
Address mask request
Query
18
0
Address mask request
Query
ICMP is used for many different functions, the most important of which is error reporting. Some of these are “port unreachable”, “host unreachable”, “network unreachable”, “destination network unknown”, and “destination host unknown”. Some not related to errors are:
Timestamp request and reply allows one system to ask another one for the current time.
Address mask and reply is used by a diskless workstation to get its subnet mask at boot time.
Echo request and echo reply is used by the ping program to test to see if another unit will respond.
Further Reading:
TCP/IP Illustrated, Volume1, The Protocols
Author:W. Richard Stevens, Publisher: Addison Wesley. ISBN 0201633469
Anthony-Claret is a software Engineer, entrepreneur and the founder of Codewit INC. Mr. Claret publishes and manages the content on Codewit Word News website and associated websites. He's a writer, IT Expert, great administrator, technology enthusiast, social media lover and all around digital guy.
User Datagram Protocol (UDP) supports the network at the transport layer. User Datagram Protocol (UDP) is an unreliable connection-less protocol and is defined by RFC 768 and 1122. It is a datagram service. There is no guarantee that the data will reach its destination. UDP is meant to provide serivce with very little transmission overhead. It adds very little to IP datapackets except for some error checking and port direction (Remember, UDP encapsulates IP packets). The following protocols or services use UDP:
– DNS
– SNMP
– BOOTP
– TFTP
– NFS
– RPC
-RIP
UDP Message Format
The UDP header includes:
Source port number (16 bits) – An optional field
Destination port number (16 bits)
UDP length (16 bits)
UDP checksum (16 bits)
This is followed by data. The UDP checksum includes UDP data, not just the header as with IP message formats. For UDP and TCP checksum calculation a 12 byte pseudo header is included which contains some fields form the IP message header. This header is not transmitted as part of UDP or TCP, but is only used to help compute the checksum as a means of being sure that the data has arrived at the correct IP address. This is the TCP/UDP pseudo header:
Source IP address (32 bits)
Destination IP address (32 bits)
blank filler(0) (8 bits)
Protocol (8 bits)
UDP length (16 bits)
Further Reading:
TCP/IP Illustrated, Volume1, The Protocols
Author:W. Richard Stevens, Publisher: Addison Wesley. ISBN 0201633469
Anthony-Claret is a software Engineer, entrepreneur and the founder of Codewit INC. Mr. Claret publishes and manages the content on Codewit Word News website and associated websites. He's a writer, IT Expert, great administrator, technology enthusiast, social media lover and all around digital guy.
Transmission Control Protocol (TCP) supports the network at the transport layer. Transmission Control Protocol (TCP) provides a reliable connection oriented service. Connection oriented means both the client and server must open the connection before data is sent. TCP is defined by RFC 793 and 1122. TCP provides:
– End to end reliability.
– Data packet re sequencing.
– Flow control.
TCP relies on the IP service at the network layer to deliver data to the host. Since IP is not reliable with regard to message quality or delivery, TCP must make provisions to be sure messages are delivered on time and correctly (Federal Express?).
– TCP Message Format
The format of the TCP header is as follows:
Source port number (16 bits)
Destination port number (16 bits)
Sequence number (32 bits) – The byte in the data stream that the first byte of this packet represents.
Acknowledgement number (32 bits) – Contains the next sequence number that the sender of the acknowledgement expects to receive which is the sequence number plus 1 (plus the number of bytes received in the last message?). This number is used only if the ACK flag is on.
Header length (4 bits) – The length of the header in 32 bit words, required since the options field is variable in length.
Reserved (6 bits)
URG (1 bit) – The urgent pointer is valid.
ACK (1 bit) – Makes the acknowledgement number valid.
PSH (1 bit) – High priority data for the application.
RST (1 bit) – Reset the connection.
SYN (1 bit) – Turned on when a connection is being established and the sequence number field will contain the initial sequence number chosen by this host for this connection.
FIN (1 bit) – The sender is done sending data.
Window size (16 bits) – The maximum number of bytes that the receiver will to accept.
TCP checksum (16 bits) – Calculated over the TCP header, data, and TCP pseudo header.
Urgent pointer (16 bits) – It is only valid if the URG bit is set. The urgent mode is a way to transmit emergency data to the other side of the connection. It must be added to the sequence number field of the segment to generate the sequence number of the last byte of urgent data.
Options (variable length
The header is followed by data. TCP data is full duplex.
Further Reading:
TCP/IP Illustrated, Volume1, The Protocols
Author:W. Richard Stevens, Publisher: Addison Wesley. ISBN 0201633469
Anthony-Claret is a software Engineer, entrepreneur and the founder of Codewit INC. Mr. Claret publishes and manages the content on Codewit Word News website and associated websites. He's a writer, IT Expert, great administrator, technology enthusiast, social media lover and all around digital guy.
Internet Protocol (IP) provides support at the network layer of the OSI model. All transport protocol data packets such as UDP or TCP are encapsulated in IP data packets to be carried from one host to another. IP is a connection-less unreliable service meaning there is no guarantee that the data will reach the intended host. The datagrams may be damaged upon arrival, out of order, or not arrive at all (Sounds like some mail services, doesn’t it?). Therefore the layers above IP such as TCP are responsible for being sure correct data is delivered. IP provides for:
Version (4 bits) – The IP protocol version, currently 4 or 6.
Header length (4 bits) – The number of 32 bit words in the header
Type of service (TOS) (8 bits) – Only 4 bits are used which are minimize delay, maximize throughput, maximize reliability, and minimize monetary cost. Only one of these bits can be on. If all bits are off, the service is normal. Some networks allow a set precedences to control priority of messages the bits are as follows:
– Bits 0-2 – Precedence.
– 111 – Network Control
– 110 – Internetwork Control
– 101 – CRITIC/ECP
– 100 – Flash Override
– 011 – Flash
– 010 – Immediate
– 001 – Priority
– 000 – Routine
– Bit 3 – A value of 0 means normal delay. A value of 1 means low delay.
– Bit 4 – Sets throughput. A value of 0 means normal and a 1 means high throughput.
– Bit 5 – A value of 0 means normal reliability and a 1 means high reliability.
– Bit 6-7 are reserved for future use.
Total length of the IP data message in bytes (16 bits)
Identification (16 bits) – Uniquely identifies each datagram. This is used to re-assemble the datagram. Each fragment of the datagram contains this same unique number.
flags (3 bits) – One bit is the more fragments bit
Bit 0 – reserved.
Bit 1 – The fragment bit. A value of 0 means the packet may be fragmented while a 1 means it cannot be fragmented. If this value is set and the packet needs further fragmentation, an ICMP error message is generated.
Bit 2 – This value is set on all fragments except the last one since a value of 0 means this is the last fragment.
Fragment offset (13 bits) – The offset in 8 byte units of this fragment from the beginning of the original datagram.
Time to live (TTL) (8 bits) – Limits the number of routers the datagram can pass through. Usually set to 32 or 64. Every time the datagram passes through a router this value is decremented by a value of one or more. This is to keep the datagram from circulating in an infinite loop forever.
Protocol (8 bits) – It identifies which protocol is encapsulated in the next data area. This is may be one or more of TCP(6), UDP(17), ICMP(1), IGMP(2), or OSPF(89). A list of these protocols and their associated numbers may be found in the /etc/protocols file on Unix or Linux systems.
Header checksum (16 bits) – For the IP header, not including the options and data.
Source IP address (32 bits) – The IP address of the card sending the data.
Destination IP address (32 bits) – The IP address of the network card the data is intended for.
Options – Options are:
Security and handling restrictions
Record route – Each router records its IP address
Time stamp – Each router records its IP address and time
Loose source routing – Specifies a set of IP addresses the datagram must go through.
Strict source routing – The datagram can go through only the IP addresses specified.
Data – Encapsulated hardware data such as ethernet data.
The message order of bits transmitted is 0-7, then 8-15, in network byte order. Fragmentation is handled at the IP network layer and the messages are reassembled when they reach their final destination. If one fragment of a datagram is lost, the entire datagram must be retransmitted. This is why fragmentation is avoided by TCP. The data on the last line, item 14, is ethernet data, or data depending on the type of physical network.
Further Reading:
TCP/IP Illustrated, Volume1, The Protocols
Author:W. Richard Stevens, Publisher: Addison Wesley. ISBN 0201633469
Anthony-Claret is a software Engineer, entrepreneur and the founder of Codewit INC. Mr. Claret publishes and manages the content on Codewit Word News website and associated websites. He's a writer, IT Expert, great administrator, technology enthusiast, social media lover and all around digital guy.
Address Resolution Protocol (ARP) provides a completely different function to the network than Reverse Address Resolution Protocol (RARP). ARP is used to resolve the ethernet address of a NIC from an IP address in order to construct an ethernet packet around an IP data packet. This must happen in order to send any data across the network. Reverse address resolution protocol (RARP) is used for diskless computers to determine their IP address using the network.
Address Resolution Protocol (ARP)
In an earlier section, there was an example where a chat program was written to communicate between two servers. To send data, the user (Tom) would type text into a dialog box, hit send and the following happened:
The program passed Tom’s typed text in a buffer, to the socket.
The data was put inside a TCP data packet with a TCP header added to the data. This header contained a source and destination port number along with some other information and a checksum.
The TCP packet was be placed inside an IP data packet with a source and destination IP address along with some other data for network management.
The IP data packet was placed inside an ethernet data packet. This data packet includes the destination and source address of the network interface cards (NIC) on the two computers. The address here is the hardware address of the respective cards and is called the MAC address.
The ethernet packet was transmitted over the network line.
With a direct connection between the two computers, the network interface card on the intended machine, recognized its address and grabbed the data.
The IP data packet was extracted from the ethernet data packet.
The TCP data packet was extracted from the IP data packet.
The data was extracted from the TCP packet and the program displayed the retrieved data (text) in the text display window for the intended recipient to read.
In step 4 above, the IP data was going to be placed inside an ethernet data packet, but the computer constructing the packet does not have the ethernet address of the recipient’s computer. The computer that is sending the data, in order to create the ethernet part of the packet, must get the ethernet hardware (MAC) address of the computer with the intended IP address. This must be accomplished before the ethernet packet can be constructed. The ethernet device driver software on the receiving computer is not programmed to look at IP addresses encased in the ethernet packet. If it did, the protocols could not be independent and changes to one would affect the other. This is where address resolution protocol (ARP) is used. Tom’s computer sends a network broadcast asking the computer that has the recipient’s IP address to send it’s ethernet address. This is done by broadcasting. The ethernet destination is set with all bits on so all ethernet cards on the network will receive the data packet. The ARP message consists of an ethernet header and ARP packet. The ethernet header contains:
A 6 byte ethernet destination address.
A 6 byte ethernet source address.
A 2 byte frame type. The frame type is 0806 hexadecimal for ARP and 8035 for RARP
The encapsulated ARP data packet contains the following:
Type of hardware address (2 bytes). 1=ethernet.
Type of protocol address being mapped( 2 bytes). 0800H (hexadecimal) = IP address.
Byte size of the hardware address (1 byte). 6
Byte size of the protocol address (1 byte). 4
Type of operation. 1 = ARP request, 2=ARP reply, 3=RARP request, 4=RARP reply.
The sender’s ethernet address (6 bytes)
The sender’s IP address (4 bytes)
The recipient’s ethernet address (6 bytes)
The recipient’s IP address (4 bytes)
When the ARP reply is sent, the recipient’s ethernet address is left blank.
In order to increase the efficiency of the network and not tie up bandwidth doing ARP broadcasting, each computer keeps a table of IP addresses and matching ethernet addresses in memory. This is called ARP cache. Before sending a broadcast, the sending computer will check to see if the information is in it’s ARP cache. If it is it will complete the ethernet data packet without an ARP broadcast. Each entry normally lasts 20 minutes after it is created. RFC 1122 specifies that it should be possible to configure the ARP cache timeout value on the host. To examine the cache on a Windows, UNIX, or Linux computer type “arp -a”.
If the receiving host is on another network, the sending computer will go through its route table and determine the correct router (A router should be between two or more networks) to send to, and it will substitute the ethernet address of the router in the ethernet message. The encased IP address will still have the intended IP address. When the router gets the message, it looks at the IP data to tell where to send the data next. If the recipient is on a network the router is connected to, it will do the ARP resolution either using it’s ARP buffer cache or broadcasting.
Reverse Address Resolution Protocol (RARP)
As mentioned earlier, reverse address resolution protocol (RARP) is used for diskless computers to determine their IP address using the network. The RARP message format is very similar to the ARP format. When the booting computer sends the broadcast ARP request, it places its own hardware address in both the sending and receiving fields in the encapsulated ARP data packet. The RARP server will fill in the correct sending and receiving IP addresses in its response to the message. This way the booting computer will know its IP address when it gets the message from the RARP server.
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Network Addressing
 IP addresses are broken into 4 octets (IPv4) separated by dots called dotted decimal notation. An octet is a byte consisting of 8 bits. The IPv4 addresses are in the following form:
192.168.10.1
There are two parts of an IP address:
Network ID
Host ID
The various classes of networks specify additional or fewer octets to designate the network ID versus the host ID.
Class
1st Octet
2nd Octet
3rd Octet
4th Octet
Net ID
Host ID
A
Net ID
Host ID
B
Net ID
Host ID
C
When a network is set up, a netmask is also specified. The netmask determines the class of the network as shown below, except for CIDR. When the netmask is setup, it specifies some number of most significant bits with a 1’s value and the rest have values of 0. The most significant part of the netmask with bits set to 1’s specifies the network address, and the lower part of the address will specify the host address. When setting addresses on a network, remember there can be no host address of 0 (no host address bits set), and there can be no host address with all bits set.
Class A-E networks
The addressing scheme for class A through E networks is shown below. Note: We use the ‘x’ character here to denote don’t care situations which includes all possible numbers at the location. It is many times used to denote networks.
Network Type
Address Range
Normal Netmask
Comments
Class A
001.x.x.x to 126.x.x.x
255.0.0.0
For very large networks
Class B
128.1.x.x to 191.254.x.x
255.255.0.0
For medium size networks
Class C
192.0.1.x to 223.255.254.x
255.255.255.0
For small networks
Class D
224.x.x.x to 239.255.255.255
Used to support multicasting
Class E
240.x.x.x to 247.255.255.255
RFCs 1518 and 1519 define a system called Classless Inter-Domain Routing (CIDR) which is used to allocate IP addresses more efficiently. This may be used with subnet masks to establish networks rather than the class system shown above. A class C subnet may be 8 bits but using CIDR, it may be 12 bits.
There are some network addresses reserved for private use by the Internet Assigned Numbers Authority (IANA) which can be hidden behind a computer which uses IP masquerading to connect the private network to the internet. There are three sets of addresses reserved. These address are shown below:Â
10.x.x.x
172.16.x.x – 172.31.x.x
192.168.x.x
Other reserved or commonly used addresses:
127.0.0.1 – The loopback interface address. All 127.x.x.x addresses are used by the loopback interface which copies data from the transmit buffer to the receive buffer of the NIC when used.
0.0.0.0 – This is reserved for hosts that don’t know their address and use BOOTP or DHCP protocols to determine their addresses.
255 – The value of 255 is never used as an address for any part of the IP address. It is reserved for broadcast addressing. Please remember, this is exclusive of CIDR. When using CIDR, all bits of the address can never be all ones.
To further illustrate, a few examples of valid and invalid addresses are listed below:
Valid addresses:
10.1.0.1 through 10.1.0.254
10.0.0.1 through 10.0.0.254
10.0.1.1 through 10.0.1.254
Invalid addresses:
10.1.0.0 – Host IP can’t be 0.
10.1.0.255 – Host IP can’t be 255.
10.123.255.4 – No network or subnet can have a value of 255.
0.12.16.89 – No Class A network can have an address of 0.
255.9.56.45 – No network address can be 255.
10.34.255.1 – No network address can be 255.
Network/Netmask specification
Sometimes you may see a network interface card (NIC) IP address specified in the following manner:
192.168.1.1/24
The first part indicates the IP address of the NIC which is “192.168.1.1” in this case. The second part “/24” indicates the netmask value meaning in this case that the first 24 bits of the netmask are set. This makes the netmask value 255.255.255.0. If the last part of the line above were “/16”, the netmask would be 255.255.0.0.
Subnet masks
Subnetting is the process of breaking down a main class A, B, or C network into subnets for routing purposes. A subnet mask is the same basic thing as a netmask with the only real difference being that you are breaking a larger organizational network into smaller parts, and each smaller section will use a different set of address numbers. This will allow network packets to be routed between subnetworks. When doing subnetting, the number of bits in the subnet mask determine the number of available subnets. Two to the power of the number of bits minus two is the number of available subnets. When setting up subnets the following must be determined:
Number of segments
Hosts per segment
Subnetting provides the following advantages:
Network traffic isolation – There is less network traffic on each subnet.
Simplified Administration – Networks may be managed independently.
Improved security – Subnets can isolate internal networks so they are not visible from external networks.
A 14 bit subnet mask on a class B network only allows 2 node addresses for WAN links. A routing algorithm like OSPF or EIGRP must be used for this approach. These protocols allow the variable length subnet masks (VLSM). RIP and IGRP don’t support this. Subnet mask information must be transmitted on the update packets for dynamic routing protocols for this to work. The router subnet mask is different than the WAN interface subnet mask.
One network ID is required by each of:
Subnet
WAN connection
One host ID is required by each of:
Each NIC on each host.
Each router interface.
Types of subnet masks:
Default – Fits into a Class A, B, or C network category
Custom – Used to break a default network such as a Class A, B, or C network into subnets.
IPv6
IPv6 is 128 bits. It has eight octet pairs, each with 16 bits and written in hexadecimal as follows:
2b63:1478:1ac5:37ef:4e8c:75df:14cd:93f2
Extension headers can be added to IPv6 for new features.
Supernetting
Supernetting is used to help make up for some of the shortage if IP addresses for the internet. It uses Classless Inter-Domain Routing (CIDR). If a business needs a specific number of IP addresses such as 1500, rather than allocating a class B set of addresses with the subnet mask of 255.255.0.0, a subnet mask of 255.255.248.0 may be allocated. Therefore the equivalent of eight class C addresses have been allocated. With supernetting, the value of 2 is not subtracted from the possible number of subnets since the router knows that these are contiguous networks. 8 times 254 = 2032
Further Reading:
TCP/IP Illustrated, Volume1, The Protocols
Author:W. Richard Stevens, Publisher: Addison Wesley. ISBN 0201633469
Anthony-Claret is a software Engineer, entrepreneur and the founder of Codewit INC. Mr. Claret publishes and manages the content on Codewit Word News website and associated websites. He's a writer, IT Expert, great administrator, technology enthusiast, social media lover and all around digital guy.
A repeater connects two segments of your network cable. It retimes and regenerates the signals to proper amplitudes and sends them to the other segments. When talking about, ethernet topology, you are probably talking about using a hub as a repeater. Repeaters require a small amount of time to regenerate the signal. This can cause a propagation delay which can affect network communication when there are several repeaters in a row. Many network architectures limit the number of repeaters that can be used in a row. Repeaters work only at the physical layer of the OSI network model.
Bridge
A bridge reads the outermost section of data on the data packet, to tell where the message is going. It reduces the traffic on other network segments, since it does not send all packets. Bridges can be programmed to reject packets from particular networks. Bridging occurs at the data link layer of the OSI model, which means the bridge cannot read IP addresses, but only the outermost hardware address of the packet. In our case the bridge can read the ethernet data which gives the hardware address of the destination address, not the IP address. Bridges forward all broadcast messages. Only a special bridge called a translation bridge will allow two networks of different architectures to be connected. Bridges do not normally allow connection of networks with different architectures. The hardware address is also called the MAC (media access control) address. To determine the network segment a MAC address belongs to, bridges use one of:
Transparent Bridging – They build a table of addresses (bridging table) as they receive packets. If the address is not in the bridging table, the packet is forwarded to all segments other than the one it came from. This type of bridge is used on ethernet networks.
Source route bridging – The source computer provides path information inside the packet. This is used on Token Ring networks
Network Router
A router is used to route data packets between two networks. It reads the information in each packet to tell where it is going. If it is destined for an immediate network it has access to, it will strip the outer packet, readdress the packet to the proper ethernet address, and transmit it on that network. If it is destined for another network and must be sent to another router, it will re-package the outer packet to be received by the next router and send it to the next router. The section on routing explains the theory behind this and how routing tables are used to help determine packet destinations. Routing occurs at the network layer of the OSI model. They can connect networks with different architectures such as Token Ring and Ethernet. Although they can transform information at the data link level, routers cannot transform information from one data format such as TCP/IP to another such as IPX/SPX. Routers do not send broadcast packets or corrupted packets. If the routing table does not indicate the proper address of a packet, the packet is discarded. Brouter
There is a device called a brouter which will function similar to a bridge for network transport protocols that are not routable, and will function as a router for routable protocols. It functions at the network and data link layers of the OSI network model. Gateway
A gateway can translate information between different network data formats or network architectures. It can translate TCP/IP to AppleTalk so computers supporting TCP/IP can communicate with Apple brand computers. Most gateways operate at the application layer, but can operate at the network or session layer of the OSI model. Gateways will start at the lower level and strip information until it gets to the required level and repackage the information and work its way back toward the hardware layer of the OSI model. To confuse issues, when talking about a router that is used to interface to another network, the word gateway is often used. This does not mean the routing machine is a gateway as defined here, although it could be.
Further Reading:
TCP/IP Illustrated, Volume1, The Protocols
Author:W. Richard Stevens, Publisher: Addison Wesley. ISBN 0201633469
Anthony-Claret is a software Engineer, entrepreneur and the founder of Codewit INC. Mr. Claret publishes and manages the content on Codewit Word News website and associated websites. He's a writer, IT Expert, great administrator, technology enthusiast, social media lover and all around digital guy.
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