VLSM

[axfalk]

We have a 129.33.1.0/24 class C network assigned to us and we needed to carve out a subnet from this network for the serial port on the router. So, I used 129.33.1.1/30 and wanted to use 129.33.1.5/24 for the rest of the network. Well, this did not work as I got the message the addresses are overlapping.I am kind of stumped since I thought the 129.33.1.1/30 subnet would include the 1-4 hosts and the 129.33.1.5/24 subnet would start with host 5.

Could anybody please shed the light as to what the problem might be?

Thanks.

[mhussein]

When you carve out a small subnet, then the largest possible subnet available would be half the original network.

You can break it down as follows:

Z.Y.1.0/30, usable ip addresses .1 - .2, broadcast .3 (.4 is outside the range of this subnet)

The next largest subnet possible available is:

Z.Y.1.128/25, usable ip addresses .129 - .254, broadcast .255 (this is the lower half of the network)

The rest of the subnet:

Z.Y.1.64/26

Z.Y.1.32/27

Z.Y.1.16/28

Z.Y.1.8/29

Z.Y.1.4/30

Note that you can not use /24 prefix anymore, otherwise it will overlap with all the subnets.

HTH,

Mustafa

[axfalk]

Thanks for your detail response. I really need to carve up only 2 addresses for my serial interface and use the rest of the addresses for the 200+ hosts I have on my network. Is this possible

[mhussein]

I can't think of any workaround. Usually it is the upstream provider's responsibility to assign a serial ip address on a seperate /30 subnet so that the customer or down-stream don't have to slice thier own address space.

If your provider won't assign you a serial ip, ask them to consider a private ip address if possible.

Other than that, you may consider using private addresses for the LAN and NAT to public ip. I don't think bridging or secondary ip addresses are good options.

Ideas, anybody?

Regards,

Mustafa

[MICHAEL BRADVICA]

Use ip unnumbered on the serial interface if the upstream provider supports it; i.e.

interface Serial2/3:0

ip unnumbered FastEthernet0/0

[mhussein]

Hey,

Thanks, I knew I was missing something, "ip unnumbered" under my nose but couldn't see it.

Kudus to you.

Regards,

Mustafa

[Richard Burts]

The solution with IP unnumbered may work. If it does not I believe that there is a solution using secondary addressing. It would not be as neat but it should work. To implement this assign the router interface an address in 129.33.1.128/25 as the primary address (this gives hosts 129 through 254). Then assign a secondary address on the router in 129.33.1.64/26 (this gives hosts 65 through 126). Then assign a secondary address on the router in 129.33.1.32/27 (this gives hosts 33 through 62). then assign a secondary address on the router in 129.33.1.16/28 (this gives hosts 17 through 30). Then assign a secondary address on the router in 129.33.1.8/29 (this gives hosts 9 through 14). You could assign a secondary address in 129.33.1.4/30 which would give you addresses 5 and 6).

This solution with secondary addresses would allow you to create non-overlapping subnets and still use all of your address space. It is a bit messy but it should work and it is a creative use of subnetting and secondary addresses.

HTH

Rick

[axfalk]

thanks to everyone for their responses. How does using secondary addressing differ from just VLSMing the 129.33.1.0/24 like this:

129.33.1.128/25

129.33.1.64/26

129.33.1.32/27

129.33.1.16/28

129.33.1.8/29

129.33.1.4/30

Thanks again

[Richard Burts]

Just VLSMing takes care of how to divide the network into non-overlapping subnets. But it does not take care of how (or where) those subnets get assigned. Secondary addressing builds on the advantage of VLSM and does address how (and where) the subnets get assigned.

HTH

Rick