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Appendix A - TCP/IP Description
Appendix A - TCP/IP (Transmission Control Protocol/Inter-
net Protocol) Description
TCP/IP is a protocol suite and related applications developed for the U.S. Department of Defense in
the 1970s and 1980s specifically to permit different types of computers to communicate and
exchange information with one another. TCP/IP is currently mandated as an official U.S. Department
of Defense protocol and is also widely used in the UNIX community.
Before you install TCP/IP on your network, you need to establish your Internet addressing strategy.
First, choose a domain name for your company. A domain name is the unique Internet name, usually
the name of your business, that identifies your company. For example, Multi-Tech’s domain name is
multitech.com ( .com indicates this is a commercial organization; .edu denotes educational
organizations, .gov denotes government organizations). Next, determine how many IP addresses
you’ll need. This depends on how many individual network segments you have, and how many
systems on each segment need to be connected to the Internet. You’ll need an IP address for each
network interface on each computer and hardware device.
IP addresses are 32 bits long and come in two types: network and host. Network addresses come in
five classes: A, B, C, D, and E. Each class of network address is allocated a certain number of host
addresses. For example, a class B network can have a maximum of 65,534 hosts, while a class C
network can have only 254. The class A and B addresses have been exhausted, and the class D
and E addresses are reserved for special use. Consequently, companies now seeking an Internet
connection are limited to class C addresses.
Early IP implementations ran on hosts commonly interconnected by Ethernet local area networks
(LAN). Every transmission on the LAN contains the local network, or medium access control (MAC),
address of the source and destination nodes. The MAC address is 48-bits in length and is non-
hierarchical; MAC addresses are never the same as IP addresses.
When a host needs to send a datagram to another host on the same network, the sending application
must know both the IP and MAC addresses of the intended receiver. Unfortunately, the IP process
may not know the MAC address of the receiver. The Address Resolution Protocol (ARP), described
in RFC 826 (located at ftp://ds.internic.net/rfc/rfc826.txt) provides a mechanism for a host to
UDP,determine a receiver’s MAC address from the IP address. In the process, the host sends an
ARP packet in a frame containing the MAC broadcast address; and then the ARP request advertises
the destination IP address and asks for the associated MAC address. The station on the LAN that
recognizes its own IP address will send an ARP response with its own MAC address. An ARP
message is carried directly in an IP datagram.
Other address resolution procedures have also been defined, including those which allow a diskless
processor to determine its IP address from its MAC address (Reverse ARP, or RARP), provides a
mapping between an IP address and a frame relay virtual circuit identifier (Inverse ARP, or InARP),
and provides a mapping between an IP address and ATM virtual path/channel identifiers (ATMARP).
The TCP/IP protocol suite comprises two protocols that correspond roughly to the OSI Transport and
Session Layers; these protocols are called the Transmission Control Protocol and the User Datagram
Protocol (UDP). Individual applications are referred to by a port identifier in TCP/UDP messages.
The port identifier and IP address together form a “socket”. Well-known port numbers on the server
side of a connection include 20 (FTP data transfer), 21 (FTP control), 23 (Telnet), 25 (SMTP), 43
(whois), 70 (Gopher), 79 (finger), and 80 (HTTP).
TCP, described in RFC 793 ( ftp://ds.internic.net/rfc/rfc793.txt) provides a virtual circuit (connection-
oriented) communication service across the network. TCP includes rules for formatting messages,
establishing and terminating virtual circuits, sequencing, flow control, and error correction. Most of
the applications in the TCP/IP suite operate over the “reliable” transport service provided by TCP.
described in RFC 768 (ftp://ds.internic.net/rfc/rfc768.txt) provides an end-to-end datagram
(connectionless) service. Some applications, such as those that involve a simple query and