Allocation of network addresses

DHCP supports three mechanisms for IP address allocation. The DHCP server can use any one or more of the of these mechanisms:

1. Automatic allocation: The DHCP server assigns a permanent IP address to a client without any manual interference. Automatic allocation is best suited for in cases where hosts are permanently connected to a network and the network does not suffer from an address shortage.
2. Manual allocation:The client's IP address is assigned manually by the network administrator. The DHCP server simply retrieves it from its storage and delivers it to the client. Manual allocation is best suited for giving IP addresses to servers of any kind. As servers are the ones to be addressed, rather than to initiate a conversation, their location should be permanent and known in the network. Manual allocation would guarantee that (although a clever use of Automatic allocation can accomplish that too).
3. Dynamic allocation: The DHCP server assigns a temporary IP address to a client without any manual interference.

Dynamic allocation is the most interesting method of the three, because it involves not only the assigning of a network address but also reclaiming and reusing of the same address by another client. Therefore, using Dynamic allocation allows for an efficient managing of a pool of network addresses and is particularly useful in cases where:

• There is a limited amount of network addresses on the net.
• The network has computers which temporarily connect and disconnect to it (e.g. portable computers) and so the network is changing frequently.

The basic mechanism for the dynamic allocation of network addresses is simple: the client requests the use of an address for a limited period of time (which is called a lease). The DHCP server allocates an address for the client, marks it as 'used' and notifies the client about the address and the lease time approved.
The client, in his turn, can:

• Extend its lease with subsequent requests.
• Ask for a permanent assignment by asking for an infinite lease.
• Release the address back to the server before the lease expires, in case it doesn't need it.

Before allocation, the server should test that the address isn't being used by another client (e.g., with an ICMP echo request). The client, when receiving the address, should test it on his side too (e.g., with ARP).

Quiz

1. Which method is most suitable for address allocation for a portable computer (one that is used in different networks)?
   • Automatic allocation.
   • Dynamic allocation.
   • Manual allocation.
2. Which method is least suitable for address allocation for your company FTP server?
   • Automatic allocation.
   • Dynamic allocation.
   • Manual allocation.

About the authors