I have a Netgear router so these instructions will have to altered to fit your particular router. Google will help with that. The concepts apply to all routers.
The router defaults to a base address of 192.168.1.1 and I'll base all addresses off that.
Also, I am at work (lunch break!) so I am doing this from memory and some of the tab names might be off a little, but if you look for them they will be close.
DHCP = Dynamic Host Configuration Protocol. It does many things but in this case it just loans out IP addresses to the devices in your house so you don't have to put static IP's on them. It makes life easier - until it fouls up.
MAC address = Media Access Control address. This is the hardware address of your device. It comes set from the factory and you don't usually set it. It looks like this: 00-2A-3B-1A-2B-3C.
On the router's admin page (http://192.168.1.1/) under the LAN configuration tab there is a section for DHCP. It lets you enable DHCP (on by default) and specify a range of addresses to hand out to devices on your network. Mine says to use a range from 192.168.1.2 to 192.168.1.32. That makes 31 addresses available to things in my house, way more than I need but it doesn't hurt to have them.
There is also a Connected Devices tab You can look in there to see the MAC address, name, and IP of all the devices currently connected to the router. It uses this information to make DHCP reservations. Take a look at the connected devices to get familiar with them.
Back on the LAN tab there is a space under the DHCP section to add reservations for devices. If you click the "Add" button it goes to another page where you can select any of the connected devices and specify what IP you want them to get. It does not have to be the one it has now, or one in the range of loaner addresses, and it shouldn't be. Pick a new one.
What I did was to start at 192.168.1.100 and assign each of my mobile devices an IP. The first was 192.168.1.100, the next was 192.168.1.101, and so on. Note that these addresses are well outside the range of DHCP addresses specified on the LAN tab. (192.168.1.2-192.168.1.32)
On the Netgear you add one address at a time and then it take you back to the LAN page.. Hit apply. The router reboots, knocking down all the existing connections, and when it comes back up and your iPad reconnects, it should show up in the Connected Devices tab with the IP you assigned to it.
Do this once for every device connected wirelessly and you are done.
As you do this you may see the number of connected devices changing. Before I fixed the addresses I could only ever see 5 connected devices instead of the 7 there should have been because my two iPads were at war with my other devices and they kept knocking each other off the router. Once I had given them all their own reserved IPs I could see all 7.
This is not a static IP, BTW. It is a loaner just like the others, but it is a reserved number that ensures that no two mobile devices will get into a conflict over a single IP address.
