ITEM: E6101L
Need help with variable subnet masking
Question:
I have a RS/6000, running on 3.2. Need help setting up variable subnet masking.
Response:
You have 2 980s with 2 ethernet cards in each, with 2 separate lans. One
980 is OK.
/etc/hosts
name IP address
1st RS txxibm1 167.99.48.221 netmask is 255.255.248.0 on all
ddpibm1 167.99.49.223
2nd RS txxibm 167.99.48.220
ddpibm 167.99.49.221
netstat -r on txxibm
167.99.48 167.99.48.220 en0
167.99.49 167.99.49.222 en0 (this should be en1)
Can't "ping ddpibm" from txxibm (which is the same machine). This is
a new installation with AIX 3.2.4.
Not using HACMP. They want to isolate traffic between the two RISCs on the
49 lan and the 48 is a regular network with lots of other machines on it (now
it's a test network). NFS is not running. routed is running (this shouldn't
be running since he has only two networks and doesn't need it).
Response:
His mask is 255.255.248.0, which uses the first 5 bits of the 3rd
octet for the network portion of the address, like this:
\<----Network----------->|\<---Host---->
1 1 1 | 1
2631 2631 2631 | 2631
84268421.84268421.84268 | 421.84268421
xxxxxxxx.xxxxxxxx.00110 | 000.xxxxxxxx Network 48
xxxxxxxx.xxxxxxxx.00110 | 001.xxxxxxxx Network 49? Nope.
49 is not different from 48 within the NETWORK portion of the address.
Unpredictable results occur when the machine's two ethernet interfaces
(en0, en1) are configured as being on the same network.
If customer wants to keep his network mask at 255.255.248.0, then his
next network up from 48 is 56, like so:
xxxxxxxx.xxxxxxxx.00111 | xxx.xxxxxxxx
Or, he can leave his host addresses alone, and change his network mask
to 255.255.255.0.
When one of the two above tasks are completed, This machine will not
need any additional static routes to reach another host on either subnet.
Hosts on either the 48 or the 56 subnet will need one additional static
route, pointing at this machine, in order to reach machines on the other
subnet.
Support Line: Need help with variable subnet masking ITEM: E6101L
Dated: November 1993 Category: N/A
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