84 | Dynamic Host Configuration Protocol (DHCP)w w w . d e l l . c o m | s u p p o r t . d e l l . c o m DHCP Packet Format and OptionsDHCP uses the user datagram protocol (UDP) as its transport protocol. The server listens on port 67 andtransmits to port 68; the client listens on port 68 and transmits to port 67. The configuration parameters arecarried as options in the DHCP packet in type, length, value (TLV) format; many options are specified inRFC 2132. To limit the number parameters that servers must provide, hosts specify the parameters thatthey require, and the server sends only those; some common options are given in Table 7-1.Figure 7-1. DHCP Packet FormatTable 7-1. Common DHCP OptionsOption Code DescriptionSubnet Mask 1 Specifies the clients subnet mask.Router 3 Specifies the router IP addresses that may serve as the client’s defaultgateway.Domain Name Server 6 Specifies the DNS servers that are available to the client.Domain Name 15 Specifies the domain name that client should use when resolving hostnamesvia DNS.IP Address Lease Time 51 Specifies the amount of time that the client is allowed to use an assigned IPaddress.DHCP Message Type 53 1: DHCPDISCOVER2: DHCPOFFER3: DHCPREQUEST4: DHCPDECLINE5: DHCPACK6: DHCPNACK7: DHCPRELEASE8: DHCPINFORMParameter Request List 55 Clients use this option to tell the server which parameters it requires. It is aseries of octets where each octet is DHCP option code.Renewal Time 58 Specifies the amount of time after the IP address is granted that the clientattempts to renew its lease with the original server.Rebinding Time 59 Specifies the amount of time after the IP address is granted that the clientattempts to renew its lease with any server, if the original server does notrespond.End 255 Signals the last option in the DHCP packet.op htype hlen hops xid secs yiaddr siaddrflags ciaddr giaddr chaddr sname optionsfileCode Length Value
