When you lease an apartment, you're assigned a mailing address that only you can use. When your lease expires, you must stop using the address, and the next person who rents the apartment inherits it. A local area network operates in a similar fashion. Each device on the network must have a unique IP address. A Dynamic Host Control Protocol, or DHCP, server leases an address to a new device for a period of time, called the DHCP lease time. When the lease expires, the DHCP server might assign the IP address to a different device.
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Every Device Needs an Address
Just as an apartment needs a mailing address to receive mail, every computer that's connected to a network needs an IP address so it can send and receive data over the network. Each network, however, has a limited number of addresses to assign to devices. The total number of addresses varies based on the network configuration. For example, many home local area networks use network addresses that range from 192.168.1.0 to 192.168.1.255. The first and last addresses are reserved for network use, which leaves 254 IP addresses for network devices.
Designating Dymanic and Static IP Addresses
A network administrator designates a portion of the available IP addresses as dynamic addresses that are assigned to devices such as computers, tablets and phones as they come and go from the network. For example, a system administrator might allocate 150 dynamic IP addresses, from 192.168.1.100 to 192.168.1.249. The remaining addresses are permanently assigned to specific devices on the network, such as routers and Web servers, that require specific IP addresses when they are connected to the network.
Assigning IP Addresses to Devices
A DHCP server tracks and manages IP address assignments on the network. When a device without a permanent assignment requests an IP address, the DHCP server assigns an address to the device for a certain period of time, such as one day. If the device is using the IP address halfway through the lease period, it requests a renewal and the DHCP server extends the lease. If the lease expires and the device hasn't contacted the DHCP server, the server recycles the IP address. Some DHCP servers wait for an additional grace period before reassigning an expired address in case the device is in a different timezone, clocks are not in sync or the device is disconnected when the lease expires.
Determining Optimum Lease Time
The network administrator determines the DHCP lease time. The optimum length depends upon the number and kinds of devices on the network. A network with more dynamic clients than IP addresses might require a shorter lease period so that addresses are recycled more frequently. A home network with many more addresses than devices might choose a longer lease period, such as one month. Many wireless routers default to a lease period of one day; Microsoft DHCP servers default to eight days.