Brian Hillis said in a message to All:

 BH>  Original message....

 BH> I don't know if this is the correct forum, but here goes anyway. 

 BH> I've always seen a net mask designated as 255.255.255.0, and
 BH> thought this was standard.  However, recent, trying to set up a
 BH> cable modem connection, the service provider has told me that the
 BH> IP is 24.112.65.3 and the net mask is 255.255.254.0.

The netmask is used to identify how many bits of the 32 bit IP address
belong to the network identification part and how many belong to the host
identification part.

In the Class C netmask of 255.255.255.0, the netmask identifies that the
first three octets (24 bits) designate the network number and the last
octet (8 bits) designates the network number. This is the
"standard" netmask only when it applies to a Class C network that
has not been subnetted.

A Class C network that has been subnetted may have subnets of:

255.255.255.252 (2 hosts -- and not functionally useful)
255.255.255.248 (4 hosts)
255.255.255.240 (8 hosts)
255.255.255.224 (16 hosts)
255.255.255.192 (32 hosts)
255.255.255.128 (64 hosts)

In all of the above cases, the first and last host is reserved for the
network number and broadcast address, respectively, which is why the 2 host
subnet is not functionally useful. 

Class A and Class B networks can be subnetted to even larger subnets.

For example, changing the netmask to 255.255.254.0 creates a slightly
larger network of 512 host addresses. (Note the network being subnetted is
a Class A network).

In this case, the first -23- bits designate the network number and the last
-9- bits designate the host number.

   [This is an extremely non-standard subnetting scheme, but leave
   it to the cable companies to reinvent networking conventions --
   perhaps they're building 500 host local loops.]

In this particular case, your host IP (24.112.65.3) is one of 510 hosts
assigned to the 24.112.64.0 network, which consists of host addresses from
24.112.64.1 through 24.112.65.254 (and the broadcast address will be
24.112.65.255).

 BH> Needless to say, I have been having a few problems, primarily
 BH> failed DNS reverse lookups for the IP number, and I keep
 BH> thinking that the net mask might be the problem. 

Most likely the failed reverse lookups are because your ISP hasn't entered
your host IP in the reverse DNS zone file.

It's highly unlikely that it's related to a netmask issue. Generally if the
netmask is configured incorrectly somewhere the misconfiguration is either
ignored or it flat out doesn't pass any IP traffic at all.

 BH> Any thoughts or comments appreciated...

Them's it. :-)


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