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Community Member

Subnetting a /21 network

Here's my best explanation of what I'm trying to get accomplished. We are a small isp with access points in 3 cities. each location is connected to internet by a T1 connection, each of the 3 locations has it's own serial interface IP. I recently acquired a /21 block of ip address ranging from x.x.240.0 - x.x.247.255 which has been designated to one of our 3 locations. What I want to do is keep 3 blocks of 254 addresses for the main location, and 2 blocks of 254 addresses for the remote locations. i.e.

(x.x.240.0 - x.x.243) for main location

(x.x.244.0 - x.x.245.0) to remote location 1 and (x.x.246.0 - x.x.247.0) to remote location 2.

What I'm needing help with is assigning the blocks of ip addresses to the remote locations. Not sure what commands to issue on the 3 routers to get this all going. Currently we are running rip only, but will be moving to OSPF soon. Thanks.

4 REPLIES
Cisco Employee

Re: Subnetting a /21 network

Not sure what you mean by assigned to remote location.

If you want users to be able to use this new block, what type of users are they ? leased line, BRI, modem, cable, dsl, ... ?

Do they have fix ip addresses or they get it from a pool on a router, or from a DHCP server ?

Do you want to use the new block with NAT ?

If you just want the router to advertise the block using rip, you can configure a static route to null like 'ip route x.x.245.0 255.255.255.0 null0' and do a 'redistribute static' inside rip (or OSPF)

Community Member

Re: Subnetting a /21 network

What I meant by "assigned to remote location" is I want to route two blocks of ip addresses to each of the remote locations serial ip interfaces so that each location can then serve the ip address blocks

Community Member

Re: Subnetting a /21 network

In addition to redistributing static, you can add loopback interfaces for each subnet you wish to advertise from a specific router and include those network addresses in the routing protocol config.

Mick.

Community Member

Re: Subnetting a /21 network

You need to do VLSM which is not supported by RIP but is by OSPF. Here would be the following blocks and masks:

x.x.240.0/22 - x.x.243.255/22 1022 hosts

x.x.244.0/23 - x.x.245.255/23 510 hosts

x.x.246.0/23 - x.x.247.255/23 510 hosts

Note that the subnet masks are different hence the need for VLSM. Also note that each location has much more than 254 hosts.If you did this:

x.x.240.0/23 - x.x.241.255/23 510 hosts

x.x.242.0/23 - x.x.243.255/23 510 hosts

x.x.244.0/23 - x.x.245.255/23 510 hosts

x.x.246.0/23 - x.x.247.255/23 510 hosts

You could run RIP because the subnet masks are the same. If the sites do not communicate with each other, then you could go with the 1st scenario. Otherwise for all sites to communicate with each other you need scenario 1 and OSPF or scenario 2 and RIP. HTH

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