The TCP/IP Guide  9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)       9  TCP/IP Network Configuration and Management Protocols (BOOTP, DHCP, SNMP and RMON)            9  Host Configuration and TCP/IP Host Configuration Protocols (BOOTP and DHCP)                 9  TCP/IP Dynamic Host Configuration Protocol (DHCP)                      9  DHCP Address Assignment and Dynamic Address Allocation and Management

DHCP Address Assignment and Dynamic Address Allocation and Management 1 2 3 4 DHCP Leases, Lease Length Policies and Management

Dynamic Allocation

While manual allocation is possible in DHCP, dynamic allocation is its real “raison d'être”. An administrator sets up a pool (usually a range or set of ranges) of IP addresses that are available for use. Each client that is configured to use DHCP contacts the server when it needs an IP address. The server keeps track of which IP addresses are already assigned, and leases one of the free addresses from the pool to the client. The server decides the amount of time that the lease will last. When the time expires, the client must either request permission to keep using the address (renewing the lease) or must get a new one. This matter of leases and how they are handled will be the subject of most of the rest of this section.

Dynamic allocation is the method used for most client machines in modern DHCP-enabled IP internetworks. It offers numerous benefits, including the following:

• Automation: Each client can be automatically assigned an IP address when it is needed with no intervention and no need for an administrator to manually decide which address goes with which client.
  
  
• Centralized Management: All the IP addresses are managed by the DHCP server. An administrator can easily look to see which devices are using which addresses and perform other network-wide maintenance tasks.
  
  
• Address Reuse and Sharing: By limiting the amount of time that each device holds an IP address, the DHCP server can ensure that the pool of IP addresses is only used by devices actively using the network. After a period of time, addresses no longer being used are returned to the pool, allowing other devices to use them. This allows an internetwork to support a total number of devices larger than the number of IP addresses available, as long as not all the devices connect to the internetwork at the same time.
  
  
• Portability and Universality: BOOTP (and DHCP manual allocation) both require that the DHCP server “know” the identity of each client that connects to it, so the server can find the client's assigned address. With dynamic allocation, there are no predefined allocations, so any client can request an IP address. This inherently makes dynamic allocation the ideal choice for supporting mobile devices that travel between networks.
  
  
• Conflict Avoidance: Since IP addresses are all assigned from a pool that is managed by the DHCP server, IP address conflicts are avoided.

The last point, of course, assumes that all the clients use DHCP. The administrator must ensure that the address pool is not used by non-DHCP devices. More about this in a later topic on DHCP address ranges and address management.

DHCP Address Assignment and Dynamic Address Allocation and Management 1 2 3 4 DHCP Leases, Lease Length Policies and Management