[NTLUG:Discuss] @home/DHCP questions: Answers

[Doug Shaw]

>If your connection is a 24/7 deal, how is it possible for your
>dynamic address to ever change?  I guess if you ever reboot your
>PC?

Caveat: I make no guarantees of the accuracy of this information, and would
ask anybody who spots errors to correct them.  Reading RFCs isn't really my
idea of a great time.
--

DHCP, despite the "dynamic" in the name, can assign what appear to be
static addresses.  That is, it can assign an IP address to the hardware
address of the network adapter, thereby ensuring that a particular network
interface "gets" the same IP address.  This is advantageous in a DSL- or
@home-style environment where you cannot always count on the customer to
know how to change that information if it's hard-coded in the OS's
configuration.  Therefore, by setting up the client machines to get their
IP addresses via DHCP, they can still get a static address, but the DHCP
server is giving them that static addressing information.

DHCP servers offer IP addresses as "leases."  The client machine sends out
a broadcast asking for IP configuration, the DHCP server responds.  If the
DHCP server is assigning bona fide dynamic addresses, it "leases" an
address to the client.  It is up to the client to behave itself with this
information; when the client sees that the lease is up - or due to be up,
depending on the configuration of the client and/or server - it is supposed
to either ask the server to "renew" its lease on its current address, or
discard the IP configuration information it has, receiving new such
information from the DHCP server.

If the server doesn't get a renewal request from the client before the
client's lease is up, it can free up that address in its database and lease
that address out to another client.  If, say, the client has been off for a
long time and missed its chances to renew its lease, it should discard its
IP addressing information and request new information from the DHCP server.

If the client doesn't "play well with others," you can have a problem where
the original client has not discarded its configuration information, but
since the lease has expired the server's gone ahead and leased that address
to another client.  Now you've got two machines trying to talk on the
network with identical IP address (the original client still THINKS it has
the address) and you've got problems.

Older TCP/IP stack and DHCP support in our favorite glass operating system
didn't always handle this addressing properly (surprise!).

To actually get around to answering the question, if the client is always
on and always has access to the DHCP server, the IP address will probably
stay the same; when the client sees that its lease is due to be up, it will
renew its lease on its current IP address.  This isn't guaranteed, though,
such as if the owners of the DHCP server are changing their distributed
range of IP addresses.  When the client says "renew me," the server will
say "sorry, can't give you that address anymore, here's your new one."

Doug