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Dual Ethernet ProxyServer User Guide
Appendix A - TCP/IP (Transmission Control Protocol/
Internet 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 (http://info.internet.isi.edu:80/in-notes/rfc/files/rfc826.txt) provides a
mechanism for a host to 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 (http://info.internet.isi.edu:80/in-notes/rfc/files/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.