RFC 1236 (rfc1236) - Page 2 of 7
IP to X
Alternative Format: Original Text Document
RFC 1236 IP to X.121 Address Mapping for DDN June 1991
To provide flexibility, Internet addresses are divided into 3 primary
classes: Class A, Class B, and Class C. These classes allow for a
large number of small and medium sized networks. The network
addresses used within the Internet in Class A, B, and C networks are
divided between Research, Defense, Government, (Non-Defense) and
Commercial uses.
As described in the MIL-STD: X25, an IP address consists of the
ASCII text string representation of four decimal numbers separated by
periods, corresponding to the four octets of a thirty-two bit
Internet address. The four decimal numbers are referred to in this
memo as network (n), host (h), logical address (l), and Interface
Message Processor (IMP) or Packet Switch Node (PSN) (i). Thus, an
Internet address maybe represented as "n.h.l.i" (Class A), "n.n.h.i"
(Class B), or "n.n.n.hi" (Class C), depending on the Internet address
class. Each of these four numbers will have either one, two, or
three decimal digits and will never have a value greater than 255.
For example, in the Class A IP address "26.9.0.122", n=26 h=9, l=0,
and i=122.
The different classes of Internet addresses [3] are illustrated
below:
Class A:
The highest-order bit is set to 0.
7-bits define the network number.
24-bits define the local address.
This allows up to 126 class A networks.
Networks 0 and 127 are reserved.
| n | h | l | i |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| NETWORK | Local Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
7 Bits 24 Bits (REST Field)
Figure 1
Class B:
The two highest-order bits are set to 1-0.
14-bits define the network number.
16-bits define the local address.
This allows up to 16,384 class B networks.
Morales & Hasse