Hi What is the use of using a DHCP server to set IP address that expires say every three days, so that the ip for the client machine changes. Is it a security issue?. Ken

This is so that an address that has been leased will not sit unused for too long. If the client does not renew within 3 days, the address will be given out to a different client. By default, the client will renew at 1/2 the lease duration. If the client is down or removed, it will obviously not renew, freeing up the address.

DHCP is one of those neat little features that Microsoft thought would improve the network. DHCP is not included in the TCP/IP white paper (standards), but most places use it because it is one less object to track. If you want to add another pc to the network, without DHCP you would have to make a note of what IP addresses where assigned to which pc's, and give the new pc the next ip address in the range. DHCP means that the computer takes care of allocating IP addresses so you can more or less forget about it. Short leases are there for two reasons. Security is one of them (you can't gain access via an ip address if the hosts ip address changes), but also to free addresses within the range for others to use. DHCP works like NT Man said. If your pc has a one hour lease, dhcp server contacts the pc after 1/2 the lease time (30 mins). If a response is given, then the lease time is reset. If there is no response, or an incorrect response, it waits for 1/2 the remaining lease (15 mins) and tries again, following the same rules. After a number of incorrect or missing responses, it free's the ip address for someone else to use. This feature is very useful if you have more pc's than ip addresses. The security aspect of it will stop trojans like netbus, sub7, back orifice etc. from hacking in. Hope that explains a lot

Yesthe best example of DHCP utilising a one to many relationship i.e dozens of clients 1 i/p address is a ISP when you connect it leases you an I/P address just for the length of time your are connected