DHCP Lease Process
DHCP Lease Process

DHCP Lease Process

A DHCP client gets an IP address from a DHCP server in four steps:

  • A DHCPDISCOVER broadcasts a request to all DHCP servers requesting an IP address.
  • The servers respond with DHCPOFFER of IP addresses and lease times.
  • The client chooses an offer and broadcasts back a DHCPREQUEST, to confirm the IP address.
  • The server handing out the IP address finish the process by returning with a DHCPACK, an acknowledgment of the request.

A client machine will attempt to renew its lease when 50% of their lease time has expired. A DHCPACK reply from the original DHCP server will also updates the information that were originally given out to the client if there are any changes, e.g. DNS server, subnet mask, etc.

If it doesn”t see the DHCP server, it will hold on to the lease for a while. When it reaches 87% of lease time, and it still can’t find the original DHCP server that issue the address, it sends a broadcast to any available DHCP server. When the lease expires and the client is still unable to get a lease, TCP/IP will be disabled for that particular workstation.

If you face some problem with the renewing process, normally releasing and renewing the IP address on the workstation by typing IPCONFIG /RELEASE followed by IPCONFIG /RENEW will solve the problem.

Most DHCP server allows you to reserve specific IP addresses for individual devices through adding reservations from the menu. You will be required to enter the 12-digit MAC address of the device needing a specific address. Reserved IP addresses will not be allocate to any other client device other than the one with the specific MAC address.

Some administrator also split the subnet into two or address pools served by different DHCP servers for load balancing and ensure that there is at least one or more DHCP server that can answer lease requests. However, administrator may have to take note that this may also reduce the size of the address pool.

In bigger environment, where there are multiple users joining and leaving the network frequently, IP address pool may be exhausted very quickly. In a typical scenario, almost all users will connect their mobile phones and laptops to the network. If the wireless network is not on a separate VLAN and there is constantly abrupt disconnection, DHCP server may not release IP addresses fast enough to re-allocate them again to new requests.