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IP Address scheme

IP Address scheme

IP Address scheme

My phone vendor will be installing a SIP based VOIP system soon and has requested that I change my internal IP address scope from class a to class c (His words). Although I am a little confused with what he has given me as a suggessted IP scheme. Currently my subnet mask is and my IP addresses start at The recommended configuration he wants is subnet mask and my address scope to start at Now it has been a while since I studied TCP/IP but isn't this basically a class c Subnet mask but still a class A address scope. I just want make sure I'm not walking off a cliff when I make the changes and have anymore downtime than needed or have to back track. Will this work? Not really seeing why a class C subnet mask and a class A address scope makes sense. Thanks for any help on this.   

RE: IP Address scheme

Both addressing schemes make some large part of the internet inaccessible to your network, needlessly.

123.x.x.x addresses out in the internet will not be reachable if you are using them locally.

RFC 1918 private address spaces exist so you do not block part of the internet.


I tried to remain child-like, all I acheived was childish.

RE: IP Address scheme

The "class A" & "class C" of the textbooks isn't quite the same as the "class A" & "class C" used on a day-to-day basis by network people.
When he said "change your scheme to class C subnets", he's telling you to change to subnets with a /24 subnet mask.
Your subnet mask is generally chosen on the basis of the number of hosts you need to cater for - a /24 subnet ("class C") can take 254 hosts, so if your office has only 100-or-so staff, this would be a good subnet size.
A /24 mask is preferable in some ways to a /23 or a /25 (etc...) because it is easier to understand for people with less training.
The kind of scheme people use would be something like this: /24 VLAN10 DATA /24 VLAN20 VOICE

RE: IP Address scheme

The choice of you're current addresses is pretty odd  as stated with the above reasons.
But unless there will be some sort of connection (for whatever reason) between your data and voice network. There is no reason to change your addressing scheme.

If there will be a connection between voice and data I can imagine the suggestion, because there probably will be some routing issues.

The other thing I can imagine is that he wants the phones in the same network as you data. This is something I STRONGLY not recommend. If you do so your voice WILL suffer due to the data traffic.
Best thing to do is create 2 vlans (voice/data) also implement QOS on the voice vlan so there is little change you're data network is interfering with the voice network.

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