RFC 1180, "TCP/IP tutorial", January 1991

Errata ID: 3456
Status: Held for Document Update
Type: Technical
Publication Format(s) : TEXT
Reported By: John Morley
Date Reported: 2013-01-15
Held for Document Update by: Brian Haberman

Section 5.4 says:

The portion of the address that is used for network number and for
host number is defined by the upper bits in the 4-byte address.  All
example IP addresses in this tutorial are of type class C, meaning
that the upper 3 bits indicate that 21 bits are the network number
and 8 bits are the host number.  This allows 2,097,152 class C
networks up to 254 hosts on each network.

It should say:

The portions of the address that are used as network number and as
host number are defined by the netmask. All example IP addresses in
this tutorial are of type class C and have a netmask of 255.255.255.0,
meaning that the first three bytes (24 bits) represent the network
number and the last eight bits the host number. This allows 16,777,216
class C networks with up to 254 hosts on each network.

Notes:

The concept of IP address loses much of its meaning if the value of the
netmask is ignored. The paragraph as originally written doesn't make
sense, in particular "the upper 3 bits indicate that 21 bits are the network
number".

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