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 Lease "Life Cycle" Overview (Allocation, Reallocation, Renewal, Rebinding and Release) and Lease Timers 1 2 3 4 DHCP Configuration and Operation

Simpler host configuration methods such as BOOTP (or DHCP manual allocation for that matter) associate a single IP address with each client machine. DHCP dynamic addressing removes this one-to-one correspondence, in favor of flexible address mapping to clients on an “as needed basis”. The clients no longer own the addresses but lease them from the true owner, the server. Obviously, then, a primary job of both a DHCP server and the administrator of that server is to maintain and manage these client addresses.

The set of all addresses that a DHCP server has available for assignment is most often called the address pool. The first issue related to address management is ensuring that the address pool is large enough to serve all the clients that will be using the server. The number of addresses required depends on a number of factors:

• Number Of Clients: Obviously.
  
  
• Stability and Frequency of Use Of Clients: If most clients are left on and connected to the network all the time, you will probably need to plan on an address for each one. In contrast, if you are serving part-time employees, or consultants who frequently travel, you can get away with sharing a smaller number of addresses.
  
  
• Consequences Of Over-Allocation: If having certain clients be unable to get a free address is a problem, you need to more carefully manage the address pool to ensure that you don't run out. If having a client not get an address is never acceptable, make sure you have as many or more addresses as clients.

I'm sure you've probably noticed that these issues are similar to those that I raised in discussing lease lengths earlier in this section. In fact, the two matters are intimately related. Generally speaking, having more addresses gives the administrator the “luxury” of using longer leases. If you are short on addresses you probably need to use shorter leases to reduce the chances of any unused addresses continuing to be allocated to devices not needing them.

DHCP Lease "Life Cycle" Overview (Allocation, Reallocation, Renewal, Rebinding and Release) and Lease Timers 1 2 3 4 DHCP Configuration and Operation