The Internet Protocol (IP) layer is the common networking and inter-
networking layer within the TCP/IP protocol suite. This allows a standard
carrier network to deliver 'datagrams' independent of the higher layer
protocols (applications), and conversely it allows an application to
communicate without any knowledge of the underlying media/network.
An internet address is a 4-octet number and is described in decimal, e.g.
129.10.2.100. There are three classes of Internet address: A, B and C.
Class A networks are usually used for very large networks that contain a
great number of hosts (or sub-networks). The first octet of the address is
used for the network number and ranges from 1 to 126. This leaves 3 octets
for host number (16 million). Class B networks are for standard large
networks and use two octets within the range 128.1 to 191.254. This leaves
two octets for hosts (65 thousand). Class C networks use three octets in the
range 192.1.1 to 223.254.254. This leaves one octet for hosts (254).
When specifying a full TCP/IP service address such as a remote login on a
host (TELNET), then a TCP port number must be specified (0-65536). The
concatenation of an IP address and TCP port number is known as a 'socket'.
For example, the standard TELNET remote login TCP port number is 23.
Therefore to fully describe the service it is necessary to specify the host IP
address and the TCP port number, e.g. 1.1.1.1 23.
Address Resolution Protocol (ARP) is used to translate IP addresses
automatically into Ethernet addresses. This protocol allows all addressing
to standardise on the IP address and removes the need for manager
configured tables. The physical station address is found by broadcasting an
ARP request to resolve the IP address. All stations on the local network
will receive the request. The station that recognises its own IP address
responds to the request indicating its Ethernet station address. The
originator of the request then uses this information to transmit a
datagram, and also keeps a record of the address for future transmissions
to the IP destination.
The TCP/IP protocol suite provides a myriad of applications, including
functions for Remote Terminal serving, File Transfer and Electronic Mail,
the most popular of which are TELNET, FTP and SMTP.
TELNET provides an ASCII Virtual Terminal interface; it allows remote
login to hosts from other hosts and/or TCP/IP terminal servers running
TELNET. To the host, the user appears to be a directly-attached ASCII
terminal. ASCII is defined and allows entry into hosts whose native
X870-300351 Issue 1 C-5 Rev.0
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