how we can tell if 2 IPs ar ein same or different broadcast doamins

Answered Question
Aug 12th, 2008

I have these two IPs

IP 1---192.168.1.1/24

IP 2-----192.168.2.1 /24

Many thanks

mahesh

I have this problem too.
0 votes
Correct Answer by Joseph W. Doherty about 8 years 4 months ago

With all the emphasis on broadcast domains and L2, but since the original question referenced IP subnets, I did want to mention broadcasts themselves are not limited to just L2. For instance, IP can "route" an IP (directed) broadcast to a subnet.

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Giuseppe Larosa Tue, 08/12/2008 - 11:08

Hello Mahesh,

the /24 notation means that the subnet mask is formed by 24 ones followed by 32-24 = 8 zeros

192.168.1.1 255.255.255.0

net part : 192.168.1.0 host part 1

the same for 192.168.2.1/24

192.168.2.1 255.255.255.0

net part : 192.168.2.0 host part 1

net 192.168.1.0 <> net 192.168.2.0

the two ip addresses are in different subnets

a broadcast domain is a layer 2 concept: it is the set of switch ports that receive the same broadcast frame.

the two hosts need a router device cannot talk directly

Hope to help

Giuseppe

Joseph W. Doherty Tue, 08/12/2008 - 11:35

You can't, unless you now whether the two subnets are virtually or physically separated, and if conneced within a routed environment, how broadcasts are forwarded, or not.

Normally, one would expect two separate subnets would not be within the same broadcast domain, but they can be.

Richard Burts Tue, 08/12/2008 - 13:11

Mahesh

I like parts of both responses. I especially like the point that Guiseppe makes that a broadcast domain is a layer 2 concept. And related to that is the point from Joseph that "You can't, unless you now whether the two subnets are virtually or physically separated". To expand on this point a bit: broadcast domains are layer 2 concepts while IP addresses and subnets are layer 3 concepts. You can not look at layer 3 IP addresses and subnets and know for sure what the layer 2 reality is.

As they both sort of say, we make assumptions about relationships between layer 2 and layer 3. One of our assumptions is that a subnet should be its own broadcast domain and when there are 2 separate IP networks there should be 2 separate broadcast domains. But it is quite possible to have a single broadcast domain (perhaps a single VLAN) and to put 2 separate IP networks into it.

It is also true that if you break some assumptions (like the assumption that a subnet should be its own broadcast domain) other assumptions about how things should work (like devices in separate subnets must have a router to communicate) may no longer be true.

HTH

Rick

Correct Answer
Joseph W. Doherty Tue, 08/12/2008 - 16:41

With all the emphasis on broadcast domains and L2, but since the original question referenced IP subnets, I did want to mention broadcasts themselves are not limited to just L2. For instance, IP can "route" an IP (directed) broadcast to a subnet.

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