Subnetting

BaldMechanic

Hello guys, brand new member!:DWorking with Wendell Odom's book on ICND1 n got stuck on subnetting. My question is this: If 10.0.0.0, for example, had already been subnetted such that a subnet - 10.153.33.0 255.255.255.0 exits, is it possible for me to pick out a further subnet from the unused addresses in this same block for a WAN link (Prolly something like 10.153.33.34/30)?

mapletune

The short to the point answer is No. Because the hosts in 10.153.33.0/24 will not be able to communicate with 10.153.33.32/30 (so, 10.153.33.33 and 10.153.33.34) They will think these 2 addresses are in their LAN segment.

A router connected to these two segments will also have trouble routing ip addresses between.

Think about it.

Routing table:

10.153.33.0/24 directly connected to interface 1
10.153.33.32/30 directly connected to interface 2

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So, I'd say it's not likely that you will encounter such a situation.

However, like Chris Bryant says, I'm sure there may be exceptions, this is networking!

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in similar spirit you could look into "route summarization." (which, incidentally, is not an English word!), supernetting, route aggregation, or other similar names.

basically, you can use a summary route of 10.153.33.0/24 on a router for everything downstream on one of it's interface.

So, maybe 10.153.33.0/25 network is downstream (10.153.33.1 ~ 10.153.33.126)
and so is 10.153.33.128/26 is also downstream (10.153.33.129 ~ 10.153.33.190)
and so is 10.153.33.192/26 is the last subnet segment downstream (10.153.33.193 ~ 10.153.33.254)

the router doesn't need 3 routes going out the same interface, just one. (won't work on the local router see?)

=p

fun eh?

her.yang

From my experience, no. The router won't let you. If you try you should receive an error message saying that the IP address you're trying to assign to the WAN interface is overlapping with an existing subnet.

sratakhin

No. However, if your subnet mask was /27 (255.255.255.224), you could use 10.153.33.0-31 for the LAN, and 33&34 for the WAN link.

mapletune

Thanks guys for clarifying.

my "suppose" you had this route table is rhetorical. It's not possible to route like that, thus the router won't let you.

BaldMechanic

Many many thanks guys, I really appreciate the lucid responses. Just started so I've obviously got a whole lot to learn. Good thing is I've seen first-hand that this community is eager to help. More dumb (or dumber) questions surely coming ur way.

BaldMechanic

MickQ

Keep them coming. That's the best way to learn.