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Chapter 11: Communications and Networking
How It Works
The process is started by a client computer, which makes a request for an IP address
to a DHCP server. If a client is new to the network, or currently has an invalid IP
address, the client will broadcast a DHCPDISCOVER message over the local subnet.
The responding DHCP server (or, in some cases, servers) will send an offer request in
the form of a DHCPOFFER packet. Then the client will acknowledge receipt of that
offer and officially ask for an address with a DHCPREQUEST packet. In return, the
DHCP server will confirm the lease and send any additional options that are config-
ured with the address inside a DHCPACK packet.
Leases are granted for a period of time known as the lease duration. After 50% of the
lease duration has lapsed, the client will request an extension—officially, this is a
lease renewal—from the DHCP server from which it originally obtained the lease. If
the client doesn’t receive a response from that server, it will wait until 87.5% of the
lease duration to attempt to renew its current lease with any DHCP machine on the
network. If no server honors the lease renewal request, the client will end its use of
the current IP address and then behave like a new client, as described previously.
Options are attributes of a DHCP lease that define certain characteristics about the IP
address and IP stack of the computer leasing the address. For example, DHCP
options specify parameters such as the DNS connection suffix (e.g., client2.has-
selltech.local), the default gateway for a particular computer (which is the router
through which traffic outside the local subnet is sent), and other important traits of a
connection. Using DHCP options saves you a lot of time in manually assigning these
traits to all your client computers, and it also adds the element of consistency—all
your computers leasing addresses within a certain scope get the same options and
not a hodgepodge of configurations.
A Windows feature called Automatic Private IP Addressing (APIPA) overlaps DHCP
functionality and can either be your best friend or drive you to insanity. Microsoft
implemented this feature so that if a client is unable to lease an IP address from a
DHCP server, it will resort to using a randomly chosen IP address from Microsoft’s
own Class B range (169.254.0.0 with subnet 255.255.0.0 and no default gateway
assigned). The address is verified using an ARP request broadcast out to the network
to ensure that no other client is using that address.
This feature is meant for convenience because most small businesses and home net-
works don’t want to offer DHCP services from Windows itself and would like their
networks to just work. However, if you have connectivity problems, Automatic Cli-
ent Configuration (ACC) can really get in the way of troubleshooting at times. It’s
best to understand ACC’s behavior under the following circumstances:
• If a client has a valid lease from a DHCP server but can’t connect to that DHCP
server, ACC will attempt to ping the default router/gateway entry defined by the