DHCPv4 assigns IPv4 addresses and other network configuration information dynamically. Because desktop clients typically make up the bulk of network nodes, DHCPv4 is an extremely useful and timesaving tool for network administrators.
A dedicated DHCPv4 server is scalable and relatively easy to manage. However, in a small branch or SOHO location, a Cisco router can be configured to provide DHCPv4 services without the need for a dedicated server. A Cisco IOS feature set (called ”Easy IP)” offers an optional, full-featured DHCPv4 server.
DHCPv4 includes three different address allocation mechanisms to provide flexibility when assigning IP addresses:
- Manual Allocation - The administrator assigns a pre-allocated IPv4 address to the client, and DHCPv4 communicates only the IPv4 address to the device.
- Automatic Allocation - DHCPv4 automatically assigns a static IPv4 address permanently to a device, selecting it from a pool of available addresses. There is no lease and the address is permanently assigned to the device.
- Dynamic Allocation - DHCPv4 dynamically assigns, or leases, an IPv4 address from a pool of addresses for a limited period of time chosen by the server, or until the client no longer needs the address.
Dynamic allocation is the most commonly used DHCPv4 mechanism and is the focus of this section. When using dynamic allocation, clients lease the information from the server for an administratively defined period, as shown in the figure. Administrators configure DHCPv4 servers to set the leases to time out at different intervals. The lease is typically anywhere from 24 hours to a week or more. When the lease expires, the client must ask for another address, although the client is typically reassigned the same address.