Subnetting question

[kerryjcox]

Quick question, I am running out of IP addresses in my 192.168.53.0 /24 subnet.  I would like to expand from a /24 to a /22 giving me IPs from 192.168.52.0 - 192.168.55.255.  Wondering if I can continue to use 192.168.53.1 as my gateway but still use the entire subnet?  If so, can I simply change the default mask on the servers behind the gate to be 255.255.252.0 and still route to the 192.168.53.1 gateway?

Thanks in advance.

KJ

[Marvin Rhoads]

That's correct.

Note that if you have DHCP at all on the network, the DHCP server parameters will need to be changed accordingly and all leases released / renewed to make sure the clients get those parameters configured properly.

The primary issue to note is that without the correct mask, hosts (including servers) will not know to use the proper broadcast address for the new network range when ARPing for a new MAC (or refreshing when an existing ARP cache entry has aged out). For statically addressed hosts, you may need to clear the ARP cache manually (or simply initiatie traffic to a given destination once the new netmask has been configured).

Of course your gateway will need to have the netmask changed as well so that it knows how to interpret the range of addresses served on that interface.

Hope this helps.

(n.b. - My 1024th post - I'm a kiloposter!)

[kerryjcox]

Understood.  I was already planning on changing the subnet mask on the DHCP server and releasing/renewing all leases. The servers would need direct intervention.  And naturally, I would change the subnet mask on the gateway itself.

I just wanted to make certain that this is a viable change, i.e. I can increase the subnet mask and still use 192.168.53.1 as the gateway.  There is athis general idea that you have to use the lowest IP within the subnet as the gateway, or 192.168.52.1, and I want to continue to use 192.168.53.1. Just want to make certain this would work.

Thanks for the input.

[Marvin Rhoads]

Yes. Sorry - I missed the 52 vs. 53 third octet distinction in your original posting.

The gateway IP can be any address (except the broadcast address) in the netblock, as long as the device interface with that address is configured and able to perform gateway functions. We typically use the lowest address in the netblock as the gateway but that's simply a common convention among network engineers to make things simpler when troubleshooting or setting things up.

That said, using 192.168.52.1 would make things more obvious in the event that anyone other than yourself (who is presumablly setting it up) comes in to manage the network and has to do anything in the middle of the night and stumbles once or twice before recalling or discovering the non-obvious gateway address. You could set up both as a transition strategy on your gateway (using a secondary address) and then decomission the original 192.168.53.1 address once all end systems were switched over.

Just my two cents.