[Gllug] DHCP/Firewalls

[David Damerell]

On Thursday, 23 Aug 2001, Paul Brazier wrote:
>> Don't forget to pick a random subnet of 192.168.* and not 
>> just 192.168.1/24.
>Why is this?

RFC1918; basically, it's to minimise the possibility of a clash if you
merge with another RFC1918 network. More common than you might think;
frex, several of the Cambridge geekhauses are now on a single VPN,
although you would never expect it to be an issue for home networking.

Since naive people will often pick 192.168.0/24 or 192.168.1/24, it's
wise to pick a range between 192.168.2-254/24, since then you won't
clash with them and are unlikely to clash with non-naive people.

10/8 is a bad choice, since you clash with anyone else using any
subset of network 10; unfortunately, enough naive people use 10/8 that
any subnet of network 10 is a bad choice, too.

Of course, 192.168/16 is an even worse choice; if you really need more
than 254 IP addresses in a private network, a /23 or /22 subnet of
192.168 or better yet one of the reserved class B [1] ranges in the
170s (selected at random) will do.

[1] Don't tell me classes are dead, you know what I mean.

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