iPad fails networking 101; how to earn it a passing grade

Soon after the first iPads started showing up on Princeton University’s network earlier this month, the university’s network admins noticed strange behavior from Apple’s tablet computer: some iPads kept using an IP address after its DHCP lease ran out. That’s part of the reason the university “banned” the iPad from its network.

What’s really going on here, and how can it be fixed?

Like virtually all Internet-capable devices, iPads obtain an IP address using the Dynamic Host Configuration Protocol (DHCP) when they connect to a WiFi network. That address isn’t exactly the iPad’s to keep, though: it comes with an expiration date. When the lease time of the address expires, which typically happens after several hours, the holder of the address must stop using it. To avoid inconvenient network interruptions, systems will contact the DHCP server and renew their address lease well before this. This doesn’t happen on the iPad—at least not when the screen is off. In the meantime, the DHCP server may have given the address in question to another system. Having two systems with the same address on the network will break connectivity for one of them—possibly both.

As long as the user is interacting with the iPad through the touchscreen, there is no problem: the iPad conforms to the DHCP protocol and renews its IP address as required. If the iPad is turned off, or WiFi is disabled, the device will disassociate from the WiFi network and stop using its IP address. But unlike the iPhone, if the iPad screen turns off—either automatically or because the user pushes the wake/sleep button—it keeps its WiFi connection alive. The lower-level IP stack also remains active, as the iPad responds to ping packets.

In this state, however, the iPad no longer pays attention to the DHCP lease time, so it keeps using its address even after the lease time expires. Waking up the iPad at this point doesn’t solve the problem, either: it keeps using the address. On the Princeton network, 41 iPads have been seen and 22 have exhibited the problem, eight to the degree of having been blocked from further access to the network.

If you experience (what may be) this problem, simply turn off WiFi on the iPad for a few moments and turn it back on. The iPad should then ask the DHCP server for a new address. There are several ways to avoid further problems. Some DHCP servers have the option of pinging addresses to make sure they’re not used before they’re given out. Setting the DHCP lease time on the server to something very high (think 24 hours) will reduce the impact of the problem.

Another solution is to give the iPad a fixed address. You can do this on the iPad in the network settings for a specific network, or on a DHCP server or home router. In the latter case, you need to associate the iPad’s MAC address (“Airport ID” or “Wi-Fi Address” in AppleSpeak, found in Settings > General > About) with a fixed address. In both cases, make sure the fixed address is outside the range of addresses given out using DHCP. For instance, DHCP may give out addresses in the range x.x.x.2 – x.x.x.200. Then give the iPad address x.x.x.201. (Your milage may vary and you may screw up your network settings, so be careful, and change network settings from a system other than the iPad itself.)