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Option Number and Description
Subnet Mask Option 1
Specifies the client’s subnet mask.
Router Option 3
Specifies the router IP addresses that may serve as the client’s default gateway.
Domain Name
Server
Option 6
Specifies the domain name servers (DNSs) that are available to the client.
Domain Name Option 15
Specifies the domain name that clients should use when resolving hostnames via DNS.
IP Address Lease
Time
Option 51
Specifies the amount of time that the client is allowed to use an assigned IP address.
DHCP Message
Type
Option 53
● 1: DHCPDISCOVER
● 2: DHCPOFFER
● 3: DHCPREQUEST
● 4: DHCPDECLINE
● 5: DHCPACK
● 6: DHCPNACK
● 7: DHCPRELEASE
● 8: DHCPINFORM
Parameter
Request List
Option 55
Clients use this option to tell the server which parameters it requires. It is a series of octets where each
octet is DHCP option code.
Renewal Time Option 58
Specifies the amount of time after the IP address is granted that the client attempts to renew its lease
with the original server.
Rebinding Time Option 59
Specifies the amount of time after the IP address is granted that the client attempts to renew its lease
with any server, if the original server does not respond.
End Option 255
Signals the last option in the DHCP packet.
Assign an IP Address using DHCP
The following section describes DHCP and the client in a network.
When a client joins a network:
1. The client initially broadcasts a DHCPDISCOVER message on the subnet to discover available DHCP servers. This message
includes the parameters that the client requires and might include suggested values for those parameters.
2. Servers unicast or broadcast a DHCPOFFER message in response to the DHCPDISCOVER that offers to the client values
for the requested parameters. Multiple servers might respond to a single DHCPDISCOVER; the client might wait a period of
time and then act on the most preferred offer.
3. The client broadcasts a DHCPREQUEST message in response to the offer, requesting the offered values.
4. After receiving a DHCPREQUEST, the server binds the clients’ unique identifier (the hardware address plus IP address) to
the accepted configuration parameters and stores the data in a database called a binding table. The server then broadcasts a
DHCPACK message, which signals to the client that it may begin using the assigned parameters.
5. When the client leaves the network, or the lease time expires, returns its IP address to the server in a DHCPRELEASE
message.
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Dynamic Host Configuration Protocol (DHCP)