Subnet with fingers
I just memorize these sequences, ungainly, but works.
Decimal masks - 128, 192, 224, 240, 248, 252, 254, 255
Wildcard masks - 127, 63, 31, 15, 7, 3, 1, 0
RFC 791 - Classful Networking
Early Internet addressing (1980s) the IP itself indicated the subnet mask, by using the High Order bits. There were only three network sizes.
/8 - Address starts with 0-127 - 128 networks
/16 - Address starts with 128-191 - 65,536 networks
/24 - Address starts with 192-223 - 16,777,216 networks
In the long ago, the hope was to use the first few bits of an address to tell the subnet mask. Even though we never do this in the modern era a few parts of classful networking are still here.
/24is a very popular prefix/16is a very popular prefix- All multicast addresses start with
1110
Internet Protocol
Specification
Addressing
To provide for flexibility in assigning address to networks and
allow for the large number of small to intermediate sized networks
the interpretation of the address field is coded to specify a small
number of networks with a large number of host, a moderate number of
networks with a moderate number of hosts, and a large number of
networks with a small number of hosts. In addition there is an
escape code for extended addressing mode.
Address Formats:
High Order Bits Format Class
--------------- ------------------------------- -----
0 7 bits of net, 24 bits of host a
10 14 bits of net, 16 bits of host b
110 21 bits of net, 8 bits of host c
111 escape to extended addressing mode
RFC1918 Dungeons
These are the most famous IPv4 networks.
RFC 1918 Address Allocation for Private Internets February 1996
3. Private Address Space
The Internet Assigned Numbers Authority (IANA) has reserved the
following three blocks of the IP address space for private internets:
10.0.0.0 - 10.255.255.255 (10/8 prefix)
172.16.0.0 - 172.31.255.255 (172.16/12 prefix)
192.168.0.0 - 192.168.255.255 (192.168/16 prefix)
We will refer to the first block as "24-bit block", the second as
"20-bit block", and to the third as "16-bit" block. Note that (in
pre-CIDR notation) the first block is nothing but a single class A
network number, while the second block is a set of 16 contiguous
class B network numbers, and third block is a set of 256 contiguous
class C network numbers.