Why are home networks prefixed with 192.168?
Why local network addresses usually are like 192.168.xxx.xxx? Is there any reason they start with 192.168 or is it just a convention?
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Convention, see RFC 1918:
Out of these blocks the 192.168.0.0/16 is most often used as the default for home routers because it consists of 256 class C networks which all have the default netmask of 255.255.255.0. The other blocks are a class A and 16 class B networks which makes them less suitable for consumer grade devices because of the different netmask required.
See also Wikipedia:Private Network which is not authoritative, but gives a nice overview.