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ssin
Asked: 2011-07-02 05:37:26 +0800 CST

Hazards of increasing network size by changing the netmask

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We are running a small network with address range 192.168.0.0/24. So we only have 254 IPs available. We want to increase this range slightly, to allow more hosts in the network. So if I change the network to 192.168.0.0/23, that should give us 256 more IP addresses right? From 192.168.0.1-192.168.1.254?

As a test I gave one of our machines on the network a static IP address 192.168.1.25/23. But this machine can't seem to connect to our default gateway, 192.168.0.1 -- the ping times out.

We have a box that serves as our DHCP, DNS, Gateway and firewall all in one, and its IP is 192.168.0.1, and on that I set the subnet mask to 255.255.254.0 (/23).

Anything I put in 192.168.1.x can't seem to get to the 192.168.0.1 gateway. Not sure what's going on. Is there something else I need to change?

Thanks in advance.

networking ip subnet netmask

2 Answers

  1. The ping won't work because the reply cannot be routed, as the router won't know where to find you 192.168.1.* machines. You'll have to change the subnet mask on all hosts in the subnet.

    That said, why bother with those tiny bitmask changes -- if you have a large subnet, just make it /16 and be done with it -- what benefits do you expect otherwise?

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  2. By setting the subnet mask to /23 instead of /24 you would in theory get 255 more IP-addresses. However, this would require you to change the subnet mask on all devices already in the network. It would also increase the number of packets sent to the common broadcast address (192.168.1.255)

    Another solution, which we use for our customers, is to add a new subnet - 192.168.1.0/24 - to the new computers, and add this to the routing table of your router. This would, if it is configured correctly, allow traffic from the 192.168.0.0/24 subnet to be sent to the 192.168.1.0/24 subnet and the other way around.

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