Ref: 99980008
Title: Specifying TCP Addresses
Date: 8/1/86

Copyright 3Com Corporation, 1991.  All rights reserved.

Specifying TCP Addresses

This section describes the rules tht govern the way TCP/IP
addresses are specified, and applies only to servers running the
TCP/IP protocols.  In general, a TCP/IP address can be expressed
as an Internet address, a physical port number, or an internet
name.

An Internet address consists of up to 12 decimal digits, which
are arranged in four fields.  Each field consists of up to 3
digits, and the fields are separated by periods:

.br;xxx.xxx.xxx.xxx

.br;Leading zeros in any single field may be omitted.

The address includes both a network number and a host number.
The actual format of these two numbers varies depending on the
class of address being designated.  Address class is determined
by the value in the first address field.  Table 3-6 lists the
valid address classes, the values used to specify them, and the
resulting address format.  The maximum value for any single field
is 255.


                    TCP/IP Address Format

                       Range of First
          Class        Address Field       Address Format*

            A             0 - 127          nnn.hhh.hhh.hhh

            B           128 - 191          nnn.nnn.hhh.hhh

            C           192 - 223          nnn.nnn.nnn.hhh


           "n" represents a digit of the network number;
           "h" represents a digit of the host number.


For example, 126.0.0.100 is a class A address, with a network
number of 126 and a host number of 0.0.100; 135.1.0.200 is a
class B address, with a network number of 135.1 and a host
number of 0.200; 192.2.3.254 is a class C address, with a network
number of 192.2.3 and a host number of 254.

The values 0 and 255 are special in TCP addressing. The value 0
means "local", as in "local host"; and the value 255 means "all",
as in "all networks".  For instance, the class B address
128.255.255.255 indicates all hosts on the network 128.  The
class A address 0.0.0.37 indicates host 37 on the local network.

The important feature of address class is the fact that the class
determines which part of the Internet address is the network
number.  In order to be able to communicate with one another, all
devices on the same network should be assigned addresses
containing indentical network numbers.  Two devices whose network
numbers are different cannot communicate with each other without
a gateway.  In addition, each host number must be unique within
its network.

The Internet address format does not provide any means of
specifying port numbers.  Therefore, the Bridge implementation
uses different Internet addresses to represent different physical
ports on a TCP server.  Up to 41 different addresses may be
assigned to a given TCP server:

* One server address.  This address is mandatory, and is used as
the source address on the active side of all connections.

* One address for each physical port.  These addresses are
optional; they are used only as destintion addresses on the
passive side of all connections and are assigned only to ports
used as circuit destinations (e.g., hosts or dial-out modems).

* One address for each rotary number, up to a maximum of eight
rotaries (numbered 129 through 135).  These addresses are also
optional, since they need to be assigned only to rotaries
actually used as circuit destinations, but not to unused rotary
numbers.

A port number consists of up to three decimal digits preceded by
an exclamation point.  Some Connection Service commands permit
either physical port numbers of Internet addresses to be
specified; others permit only physical port numbers, but not
Internet addresses.  Similarly, some command displays (e.g., SHow
ADDRess) include only physical port numbers, but not Internet
addresses.  The command and display descriptions indicate when
only physical port numbers apply.

An internet name consists of a simple character string.  The
maximum length of the internet name is 40 alphanumeric
characters, the first character of which must be alphabetic.
Subsequent characters may be either alphabetic or numberic.  The
nonalphanumeric characters permitted are the underscore (_), the
period (.), and the dash (-).

In the SHow NAmes command, which displays the Internet address
associated with an internet name, an asterisk (*) may be
substituted for all or part of the name.  The asterisk
represents a wildcard character, and indicates "any value".
