Subnets vs Broadcast domains

b.budd · ‎02-15-2008

Do secondary IP addresses on a router interface create, in effect, separate broadcast domains?

For example (using 24-bit masks):

interface etherthernet0

ip address 192.168.1.1 secondary

ip address 10.1.1.1

Are PCs in the 192.168 subnet considered to be in a different broadcast domain than PCs in the 10.1 subnet? If so, then does that mean that broadcasts generated by devices in 192.168 will not be propagated across switches to devices in the 10.1?

Thanks

lamav · ‎02-15-2008

Hi:

A subnet is a broadcast domain.

More specifically, by default, a router will not forward a Layer 3 broadcast outisde the subnet. That's why, for example, when you create a user VLAN on a layer 3 switch and the DHCP server does NOT reside in the local VLAN, a "helper address" must be configured for that user VLAN. The router/L3 switch will NOT forward the DHCP Request broadcast from the end machine past the router's L3 boundary, so it will act as sort of a proxy and make the request to the DHCP server for the end user.

So, if I subnet 10.10.10.0 with a mask of 255.255.255.240 (/28), the first 2 subnets that would be created are:

1. 10.10.10.0

2. 10.10.10.16

For the 10.10.10.0 subnet, there will be 14 host addresses (10.10.10.1 - 14) with a broadcast address FOR THAT SUBNET ONLY of 10.10.10.15.

For the 10.10.10.16 subnet, there will be 14 usable host addresses, too, (10.10.10.17-10.10.10.30), and a braodcast address FOR THAT SUBNET ONLY of 10.10.10.31.

Hope my rambling helps...:-)

Victor

massimiliano.serafino · ‎02-15-2008

Hi,

VLANs are separate broadcast domain, not subnets. Infact you can have 2 subnets on the same VLAN. The best practice is to have different subnets for different VLAN.

If you have 192.168.1.0/24 and 10.1.1.0/24 on the same VLAN, subnets are on the same broadcast domain. If 192.168.1.0/24 is in the VLAN A e 10.1.1.0/24 on VLAN B they are in different broadcast domains.

The answer to your question is: secondary IP addresses on a router interface don't create separate broadcast domains.

I hope this helps.

Best regards.

Massimiliano.

Edison Ortiz · ‎02-15-2008

> Do secondary IP addresses on a router interface create, in effect, separate broadcast domains?

Nope.

> Are PCs in the 192.168 subnet considered to be in a different broadcast domain than PCs in the 10.1 subnet?

Different IP subnet, same broadcast domain. All devices on that broadcast domain will listen to the same broadcast regardless their IP subnet. You need a switch to break IP subnets segments located in the same geographic location in order to have different broadcast domains.

> If so, then does that mean that broadcasts generated by devices in 192.168 will not be propagated across switches to devices in the 10.1?

Everyone on that broadcast domain will get it.

__

Edison.

lamav · ‎02-15-2008

"Different IP subnet, same broadcast domain."

Edison, so what you are saying is that a user on subnet 10.10.1.0/24 (Subnet A) and another on 10.10.2.0/24 (Subnet B) -- lets say these are 2 subnets hanging off of ethernet0 and ethernet1, respectively, of the same router -- are on different subnets but the same broadcast domain?

If so, do you mean that all users on Subnet B are going to receive the DHCP Request broadcast of a user on subnet A?

Edison Ortiz · ‎02-15-2008

keyword router. The router breaks the broadcast domain as broadcast packets aren't routed.

Now, if the router is connecting E0 on Switch A and E1 on Switch A and both switchports are configured for the same Vlan, then both IP subnets belong to the same broadcast domain.

lamav · ‎02-15-2008

Agreed.

The key is that the IP subnets are indeed separate broadcast domains by default, and that they must be in the same VLAN to be considered to be part of the same broadcast domain.

When I gave my example above to Roseborough, I should have made clear that the underlying assumption of my explanation was that the VLANs I suggested have a single subnet in them.

Thanks

Edison Ortiz · ‎02-15-2008

Based on the OP configuration (using secondary IP address), they are going to be in the same Vlan.

Different Vlan configuration requires dot1q trunking on the router ethernet interface.

lamav · ‎02-15-2008

"Based on the OP configuration (using secondary IP address), they are going to be in the same Vlan."

Yup.