somebody asked about this, so i knocked out this quick email:

The netmask tells the tcp stack which range of ip’s are on the local

network, i.e. any packets for ip’s in this range can jut be dumped

onto the local net and the other machine will find them. Conversely

anything outside the netmask should be sent to the default router.

If you had:

box A:
box B:

Assuming they were cabled correctly, they would just find each other,

because the netmask of says -

is on the local net.

If you changed box B to it wouldn’t know how to

reach box A anymore and would therefore send all packets to its default gateway

How they get their IP addresses is up to you, with 2 machines the


is to pick some yourself and assign them. If they are not connected to

the internet then you can pick anything you like but they probably


be from the non-routable sets (see below). If you have a few machines


could use DHCP to allocate auto-magically at boot time.

from rfc 1918….

3. Private Address Space

The Internet Assigned Numbers Authority (IANA) has reserved the
following three blocks of the IP address space for
private internets:        -  (10/8 prefix)      -  (172.16/12 prefix)     - (192.168/16 prefix)