{"id":312,"date":"2005-07-21T09:57:00","date_gmt":"2005-07-21T09:57:00","guid":{"rendered":"http:\/\/markwilson.me.uk\/blog\/2005\/07\/best-practices-for-managing-automatic.htm"},"modified":"2007-07-16T23:40:49","modified_gmt":"2007-07-16T22:40:49","slug":"best-practices-for-managing-automatic","status":"publish","type":"post","link":"https:\/\/www.markwilson.co.uk\/blog\/2005\/07\/best-practices-for-managing-automatic.htm","title":{"rendered":"Best practices for managing automatic IP addressing with DHCP"},"content":{"rendered":"

Dynamic host configuration protocol<\/a> (DHCP) is often taken for granted – we expect it to work; however there are a few items which need to be considered and this post is intended as a general discussion of DHCP best practice.<\/p>\n

Most administrators will be familiar with the overall DHCP concept – basically a database of IP addresses allocated to clients dynamically, allowing centralised IP address management; however, most of the organisations I see still need to use static addresses for some devices (e.g. servers). Whilst there is nothing wrong with this and I would still suggest using fixed IP addresses for networking equipment and the DHCP server itself, reservations can be useful to reserve particular addresses for certain clients, based on their media access control (MAC) address. The main drawback of this approach is that if the NIC in the computer changes, so does the MAC, although reprogramming the MAC address is possible (as is setting up a new reservation).<\/p>\n

If there are static addresses in use which fall within the an IP address range intended for DHCP, exclusions can be configured (much easier than configuring several scopes to cover the fragmented IP range). Exclusions can be configured for a single address, or for a range of IP addresses.<\/p>\n

Lease duration is another area to consider (i.e. the amount of time before a client needs to renew its DHCP address) – if this is set too long, and there are a large number of mobile clients, there is a risk of running out of available IP addresses as these mobile clients join the network, lease an address and then leave again without releasing it; conversely, too short and there is a large amount of renewal traffic as the DHCP client attempts to renew its lease at the half life. For most environments, I find that an 80:20 rule can be applied – i.e. provide 20% more addresses than are expected to be in use at any one time (to cater for mobile clients) and set the lease time to 1 day but for a subnet with largely static PCs, then longer leases may be appropriate.<\/p>\n

DHCP includes a number of pre-defined options that can be set on a client:<\/p>\n