Q&A: Using the same IP address

Q We have several systems on our network now, centred on a BT Infinity broadband connection, including my iMac, two MacBooks, a couple of iPads, iPhones, Blackberries, and a Time Capsule. However about half the time that I start my iMac up, it reports that something else is using the same IP address, and I lose access to my wireless network. Printing then fails too. How can I make this network more reliable?

A You almost certainly have two DHCP servers running at the same time on your network: one in your Infinity router, the other in your Time Capsule. Thus when you start your iMac up and it asks for an IP address to be assigned to it, either server could lease it an IP address, which could clash with one already assigned by the other DHCP server.

You must turn off one or other of the DHCP servers. This leaves either the router or Time Capsule serving IP addresses; it does not matter which so long as it remains permanently switched on, more likely your broadband router.

Indeed a better plan would be to use DHCP only for those devices that are transiently on the network, such as the phones, and assign fixed IP addresses to all your other devices, including the iMac and MacBooks. Then when you have network problems you will know by the IP address affected exactly which is in trouble.

Take all other devices off your network, apart from your iMac, router, and Time Capsule. Referring to its documentation, point your browser at the router, log onto it, and set its internal IP address to end in .253, keeping the rest of its original address, e.g. Configure its DHCP server to hand out addresses only in the range .50 to .240.

Open the Network pane on the iMac, and set it to use a fixed IP address of, say,, with a subnet mask of Put the router’s IP address first in its DNS Servers, followed by those given by your ISP, or OpenDNS if you prefer.

Using Airport Utility, connect to the Time Capsule, turn its DHCP server off, and assign it a fixed IP address of, say, Then start up your MacBooks and assign then addresses of, say, and .7, and so on. Mobile devices can be left to obtain their addresses from the now lone DHCP server.

Updated from the original, which was first published in MacUser volume 28 issue 16, 2012.