4.6 NAT and DHCP

By default, the AirPort acts as both a NAT server and a DHCP server for your wireless clients.^[3] DHCP service is controlled by the DHCP Functions tab. To turn DHCP on, check the Provide DHCP address delivery to wireless hosts box. You can specify the range of IPs to issue; by default, the AirPort hands out leases between 10.0.1.2 and 10.0.1.50. You can also set a lease time here. The lease time specifies the lifetime (in seconds) of an issued IP address. After this timer expires, the client reconnects to the DHCP server and requests another lease. The default of 0 (or unlimited) is probably fine for most installations, but you may want to set it shorter if you have a large number of clients trying to connect to your AirPort.

^[3] If you're just joining us, NAT and DHCP stand for Network Address Translation and Dynamic Host Configuration Protocol, respectively. See Chapter 3 for more details.

If you don't have another DHCP server on your network, the AirPort can provide service for your wired hosts as well. Check the Distribute addresses on Ethernet port, too box if you want this functionality.

Check this box only if you don't have another DHCP server on your network! More than one DHCP server on the same subnet is a bad thing, and will bring the wrath of the sysadmin down upon you. Watching two DHCP servers duke out who gets to serve leases may be fun in your spare time, but can take down an entire network, and leave you wondering where your job went. What were you doing connecting unauthorized gear to the company network, anyway?

If you have more than one AirPort on the same wired network, make sure that you enable DHCP to the wire on only one of them and, again, only if you don't already have a DHCP server.

NAT is very handy if you don't have many IP addresses to spare (and these days, few people do). It also gives your wireless clients some protection from the wired network, as it acts as an effective one-way firewall (see Chapter 3 for the full story of NAT and DHCP). In the Configurator, NAT is set up in the Bridging Functions tab. To enable NAT, click the Provide network address translation (NAT) radio button. You can either specify your own private address and netmask, or leave the default (10.0.1.1 / 255.255.255.0).