TCP/IP Tutorial and Technical Overview - IBM Redbooks

administration workload and removes the benefit of recycling of addresses.
Unauthorized DHCP servers can also be set up, sending false and potentially
disruptive information to clients.
► In a DHCP environment where automatic or dynamic address allocation is
used, it is generally not possible to predetermine the IP address of a client at
any particular point in time. In this case, if static DNS servers are also used,
the DNS servers will not likely contain valid host name to IP address
mappings for the clients. If having client entries in the DNS is important for the
network, you can use DHCP to manually assign IP addresses to those clients
and then administer the client mappings in the DNS accordingly.
3.7.8 BOOTP and DHCP interoperability
The format of DHCP messages is based on the format of BOOTP messages,
which enables BOOTP and DHCP clients to interoperate in certain
circumstances. Every DHCP message contains a DHCP message type (51)
option. Any message without this option is assumed to be from a BOOTP client.
Support for BOOTP clients at a DHCP server must be configured by a system
administrator, if required. The DHCP server responds to BOOTPREQUEST
messages with BOOTPREPLY, rather than DHCPOFFER. Any DHCP server
that is not configured in this way will discard any BOOTPREQUEST frames sent
to it. A DHCP server can offer static addresses, or automatic addresses (from its
pool of unassigned addresses), to a BOOTP client (although not all BOOTP
implementations will understand automatic addresses). If an automatic address
is offered to a BOOTP client, that address must have an infinite lease time,
because the client will not understand the DHCP lease mechanism.
DHCP messages can be forwarded by routers configured as BOOTP relay
agents.
3.8 RFCs relevant to this chapter
The following RFCs provide detailed information about the connection protocols
and architectures presented throughout this chapter:
► RFC 791 – Internet Protocol (September 1981)
► RFC 792 – Internet Control Message Protocol (September 1981)
► RFC 826 – Ethernet Address Resolution Protocol: Or converting network
protocol addresses to 48.bit Ethernet address for transmission on Ethernet
hardware (November 1982)
► RFC 903 – A Reverse Address Resolution Protocol (June 1984)
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