SUMMARY: Different Subnet Masks

From: Farzad Mansour (
Date: Mon May 19 1997 - 11:50:21 CDT

Original Posting:
> I have question on subneting. If there is an incorrect, well I can't say
> incorect, I say different subnetmasks for 2 hosts on the same physical
> lan, what things will exactly work and what things will not work. It
> sounds like your broadcast and network will get hosed somewhat. Any
> indications will be appreciated.


Thanks to: Arthur Hyun David M. Davisson Mike Salehi Richard Schulting Satish I got my answer. The most complete was from Arthur Hyun:

the broadcast address is derived from the network and the subnet value, so if the subnet values are different the broadcast addresses will often be different (but not always!). if the broadcasts are different, then hosts will not recognize each other's IP broadcasts. this will break things like rwho, rwall, bootp/dhcp when not configured to use a specific server, and stuff like that.

other than that, the subnet is used for IP routing. the machine checks to see if a destination IP address is on your network. if they are the same, the machine will send directly. if not, it will try to send to a router. this can cause extra traffic on your LAN and it will show up in a traceroute. your router may also send an ICMP redirect to the sending machine, and that will show up in the host's routing table.

note that different netmasks does not necessarily mean disjoint networks. one netmask may be bigger than the other, so one host may think it has a local route whereas the other may not.


Other answers: ============================================================ On 16 May 97 at 10:55, Farzad Mansour wrote:

Properly setup differnet subnets on the same physical LAN will not hose anything. If you can describe your problem in more detail I can give you some guidance.

Other answers: ============================================================ Farzad,

The result is not as bad as in theory can be. Some programs (nfsd) will do the right thing. On the otherhand some others will not arp properly and you need a route or arp proxy between the two.

Other answers: ============================================================= first, one shouldn't do this. Many problems arise with conflicting netmasks.

some examples: Default gateway isn't reagable probably RIP routing doesn't work properly peer to peer normaly doesn't as well

If you would like to find out exactly what addresses are able to communicate, check the amount of zero-bits in de mask (write them down binary) and put your host adresses beside them (binary as well)

again, I wouldn't do this...



Other answers: ============================================================= Don't think you can different subnet masks on machines that reside on the same segment! ...assuming your are running Ethernet (IEEE) tcp/ip.


Captain Zod...

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