Someone asked me to review a tutorial they were writing on internet addresses which lead me to put a section from "man inet" on my scratchpad (it's been there for months):

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Internet Addresses:

Values specified using dot notation take one of the following forms:

a.b.c.d
a.b.c
a.b
a

When four parts are specified, each is interpreted as a byte of data and assigned, from left to right, to the four bytes of an Internet address.

When a three-part address is specified, the last part is interpreted as a 16-bit quantity and placed in the right-most two bytes of the network address. This makes the three-part address format convenient for specifying Class B network addresses as in 128.net.host.

When a two-part address is supplied, the last part is interpreted as a 24-bit quantity and placed in the right-most three bytes of the network address. This makes the two-part address format convenient for specifying Class A network addresses as in net.host.

When only one part is given, the value is stored directly in the network address without any byte rearrangement.

All numbers supplied as parts in dot notation can be decimal, octal, or hexadecimal, as specified in the C language (i.e., a leading 0x or 0X implies hexadecimal; a leading 0 implies octal; otherwise, the number is interpreted as decimal).

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This is from a pretty old Unix system (HP-UX).

                - tye