User guide

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Appendix A - TCP/IP Description
Multi-Tech Systems, Inc. MultiVOIP 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 will 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 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).