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So I have a bunch of switches which connect up some PCs on a data network and IP Phones and other devices on a telephone network

Devices on the data network are configured both statically and via DHCP to use to following range: Network: / 24 Gateway: (Cisco ADSL router - provides access to the internet)

The IP Phones are all configured statically to use the following range: Network: / 24 Gateway: None

Now if you are on the data network, and you want to connect to a device on the telephone network, you have to change you have IP from 50.x network to the 51.x network.

What I'd like to know is how do you achieve communication without needing to change IPs? All the data for both networks runs through the same switches. The switches are currently sitting on the 50.x network.

I presume we would need a router, or some sort of routing device or rule? This seems overkill though because all the devices are connected to the same switches, and are in the same broadcast domain.

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So you separate those 2 network using VLAN? (since you mentioned both networks run on the same hardware(s)). – Darius Oct 9 '13 at 7:14
up vote 4 down vote accepted

I'm an amateur when it comes to networking, but I think you can do this by setting a proper netmask to cover a wide range of IPs. Perhaps to your main router between the two networks?

It's similar to "". You can use a subnet mask calculator (on Linux you have gip program) or one online such as:

Address:    11000000.10101000.00110010.00000000
Netmask:   11111111.11111111.11111110.00000000
Wildcard:   00000000.00000000.00000001.11111111
Network Address: / 23   11000000.10101000.00110010.00000000
Broadcast Address:  11000000.10101000.00110011.11111111
First host:    11000000.10101000.00110010.00000001
Last host:  11000000.10101000.00110011.11111110
Total host count:   510 
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This looks promising. If I implement it on a live network, will it break? Each statically-assigned device will have to be reconfigured manually, which will take time. This could become problematic during the transition phase. Can a 50.x /24 network communicate directly with a 50.x /23 network? Can I implement this in a phased approach, or do I have to do it all at once in a big bang? The reason I ask is because it will have to be done during business hours, so that 3rd party suppliers can reconfigure their devices. – Saajid Ismail Oct 9 '13 at 9:30
I suggest to test this before you go live. All you need are 3 routers and 2 PCs to see if they interact with each other. Lastly, I don't understand why you've set up the devices statically, since routers nowadays can use DHCP and set / reserve a permanent IP address for a specific MAC-address (which is unique for each network device). – medigeek Oct 9 '13 at 11:11
Servers, switches, IP phones, IP base stations, PBX, network printers, etc are all configured with static addresses on the client itself, because DHCP reservations only work if the DHCP server is online and working. I can't risk these devices being kicked off the network due to the DHCP server being down, or losing its settings, or being hit by a surge. Other clients - mobile phones, tablets, desktops and laptops all receive DHCP-assigned addresses. The way I see it, every network has to be designed with at least a few static addresses for core equipment. – Saajid Ismail Oct 9 '13 at 13:09

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