broadcasts and subnetting

gojericho0

Lets say we have a WAN that connects four LAN’s in different buildings w/ no VLANS on the LANS. The entire WAN uses the network address 192.168.0.0 with a default subnet mask 255.255.0.0

Would the routers forward broadcast traffic in this case since there are no subnets or VLANS?

Webmaster

The default subnet would be 255.255.255.0. Regardless, that's not a realistic scenario. A router is for routing traffic between subnets, so each 'side' of the router would have a different (sub-)network address. The router would not forward broadcast, nor would it be able to work at all.

gojericho0

so inorder to subnet would you have to have a bunch of routers on the same LAN?

gojericho0

nevermind i'm getting myself confused.

I'm just trying to figure out how exactly subnetting lessens broadcast traffic. I am just thinking one LAN has one router. Lets say its the 192 class network and that 100 nodes are on the network. If a broadcast goes out would it generate traffic for 254 nodes? and with subnetting you could potentially set the network to 127 nodes reducing your broadcast by half.

schwarztrader

Ouch! now i'm getting confused, lol. JK, maybe the webmaster should set you straight regarding subnetting as I almost took a crack at it but I don't know if I can untangle this web.

What information have you read on subnetting so far? I highly recommend reading the Cisco Press books regarding subnetting. Heck, I've read it almost 3 times so far.

gojericho0

I'm am using the Sybex book 4th edition and have read basically how subnetting works by dividing a big network a bunch of smaller networks inorder to make it easier to manage and cause less broadcast traffic.

I am pretty good at doing the calculations for subnetting and determining how many host bits you want to borrow.

What is really confusing me is that it seems the only way you could subnet would be to do it is to have a WAN made up or a bunch of LANS and that each LAN would be considered a subnetwork.

The thing that confuses me is that if routers block broadcasts what does it matter if they are subnetted or not. The only good reason I was thinking was my last post above. that instead of broadcasting messages to hosts 1-254 (even though only 100 hosts might be on that LAN) a subnet could break it down to only broadcasting to 128 hosts which would be a lot close to 100 and maybe reducing network traffic that way.

Does this make sense to anyone except my crazy mind haha

Webmaster

Perhaps a refresher of this material will work: www.techexams.net/technotes/networkplus/networkcomponents.shtml

The thing that confuses me is that if routers block broadcasts what does it matter if they are subnetted or not.

It matters only if you want the nodes in the networks on either side of the router to actually communicate with eachother. A router is used to connect for example two IP (sub-)networks. The subnets are the reason for the router to block broadcast from one network from entering into anotehr. Broadcasts in one IP subnet are not addressed to nodes in another IP subnet, so should not, and are not, forwarded by the router (by default).

gojericho0

Your explination makes more sense. I just want to make sure I have it down.
Two LAN's that make up a WAN would be required to have subnets else computerA from LAN1 would not be able to communicate with computerB on LAN2 directly because they would be on two different private networks and not on one network thats been subnetworked.

Webmaster

gojericho0 wrote:

Your explination makes more sense. I just want to make sure I have it down.
Two LAN's that make up a WAN would be required to have subnets else computerA from LAN1 would not be able to communicate with computerB on LAN2 directly because they would be on two different private networks and not on one network thats been subnetworked.

Not entirely. Two lans that make up a WAN by connecting them to eachother with a router automatically are two different subnets. Since the two LANs are in the same routed WAN they have to use a different IP range (unless NAT is used), because each node in the routed WAN needs a unique address. The routers would basically treat an 'IP subnet' the same way as an entire 'IP network'.

Definitely not my best explanation, I need to get some sleep, the sun starting to rise again

gabilan

I think that gojericho0 should review the concepts of VLAN, Broadcast domain and collision domain.

Drakonblayde

Basically, each interface on a router represents a different subnet. The routers job is to allow communication between subnets.

Subnetting to break a broadcast domain is simple....


Let's say you've got 500 nodes in the building split among 4 floors, each floor is it's own department. Yeah, you've got some serious port density in your wiring closet.

Now, let's say one node broadcasts...

All of the other 499 nodes are going to receive and process that broadcast when it probably only needed a response from a machine on it's own floor.

So you throw a router into the wiring closet (or setup a Vlan, whatever), break the network up by floor and subnet your address space. Now when one node broadcasts, it's only going to be seen by 124 other nodes. This cuts down on network traffic, making your network less congested and helping your latency.

It's a bit simplistic in it's explanation and there's more to it, but that's how it works in a nutshell.

Routers are the boundaries of your LAN. You normally don't have more than one router with an IP in the same LAN (after all, it's pointless, your computer only sets one default gateway with no backup, unlike the DNS servers, so there's nothing to failover too... if the gateway goes down, traffic isn't leaving your subnet)

But your initial scenario is totally unrealistic, if you're connecting different buildings and using routers you have to subnet or you're not getting any internetworking done. That's a routers entire purpose, to facilitate communication between networks, not within the same network, that's what your switches and hubs are for.

gojericho0

Thanks guys, much more clear now. It's hard when you don't have any real work experience and are trying to picture how these things actually work. I appreciate all the responses