Overview
The Dynamic Host Configuration Protocol (DHCP) is a network protocol that enables a server to automate
assignment of IP addresses to hosts. A DHCP server can be configured to provide other network information like
IP addresses of TFTP servers, DNS server, boot file name and vendor specific options. Commonly there are two
types of address assignments, dynamic and manual. The lease of dynamic addresses is renewed periodically;
manual leases are permanently assigned to hosts. With this feature, you can configure multiple pools of IP
addresses for IP address assignment and tracking.
IP pools
A DHCP server is configured with IP pools. The server is then instructed to use IP addresses falling into the
specified range of IP while offering leases. Multiple IP pools are configured to not have duplicate or overlapping IP
subnets. You can also configure a DHCP server with multiple IP ranges within an IP subnet; this confines the
allocatable IP addresses within the configured IP pool.
An IP pool will be claimed valid only if it is either:
• Dynamic pool – Has a network address, subnet mask and IP range(s)
• Static pool – Should have a static IP-to-MAC binding.
The DHCP server will discard the invalid and incomplete pools and will only operate on the valid IP pools. The
DHCP server will require at least one valid pool to start.
DHCP options
On a DHCP server, an IP pool is configured with various options. These options signify additional information
about the network. Options are supported with explicit commands such as
boot-file
. Option codes that
correspond to explicit commands can not be configured with a generic option command; the generic option
command requires an option code and TLV.
NOTE:
RFC 2132 defines various network information that a client may request when trying to get the lease.
BootP support
The DHCP server also functions as BootP server. A manual binding configured in a static IP Pool may either
service a BootP client request or a DHCP client request.
Authoritative server and support for DHCP inform packets
The server message
DHCPinform
may be received when the server is already configured for static IPv4
addresses so that the server can to get configuration parameters dynamically.
NOTE:
RFC 2131 states that if a client has obtained a network address through some other means
(e.g., manual configuration), it may use a
DHCPinform
request message to obtain other local
configuration parameters. Servers receiving a
DHCPinform
message construct a
DHCPACK
message with any local configuration parameters appropriate for the client without: allocating a new
address, checking for an existing binding, filling in
yiaddr
or including lease time parameters.
Chapter 8
DHCPv4 server
Chapter 8 DHCPv4 server
257
