The DHCP process allows devices to request and obtain IP addresses from a DHCP server. The server assigns IP addresses, subnet masks, default gateways and DNS servers to clients. Assigned addresses expire after a lease period and clients must renegotiate with the server. DHCP operations involve clients broadcasting requests, servers responding with offers, clients selecting an offer and acknowledging the assignment.

1. DHCP Service

2. Overview
• The Dynamic Host Configuration Protocol (DHCP) is
a set of rules used by a communications device such as
a computer, router or network adapter to allow the device
to request and obtain an IP address from a server which
has a list of addresses available for assignment.
• IP addresses and other parameters such as the default
gateway, subnet mask, and IP addresses of DNS servers
can be assigned by the DHCP server.
• The DHCP server ensures that all IP addresses are
unique.
• DHCP functionally became a successor to the older
BOOTP (Bootstrap protocol), whose leases were given
for infinite time and does not support options (DNS,
Router, addresses).

3. DHCP Process
• IP address assignment occurs when the DHCP client
boots up or regains connectivity to the network.
• The DHCP client sends out a query requesting a
response from a DHCP server.
• The DHCP server then replies to the client with its
assigned IP address, subnet mask, DNS server and
default gateway information.
• The assigned IP address generally expires after a
predetermined period of time, at which point the DHCP
client and server renegotiate a new IP address from the
server's predefined pool of addresses.
• Office networks should use DHCP, in particular when
workers make extensive use of laptops which link
directly to the network occasionally.

4. DHCP operations
• DHCP operations fall
into four basic phases.
These phases are
– IP lease request,
– IP lease offer,
– IP lease selection, and
– IP lease
acknowledgement.

5. DHCP discovery
• The client broadcasts on the local physical
subnet to find available servers.
• This client-implementation creates a UDP
packet with the broadcast destination of
255.255.255.255 or subnet broadcast
address.

6. DHCP offers
• When a DHCP server receives an IP lease
request from a client, it extends an IP lease
offer.
• This is done by reserving an IP address for the
client and broadcasting a DHCPOFFER
message across the network.
• This message contains the client's MAC
address, followed by the IP address that the
server is offering, the subnet mask, the lease
duration, and the IP address of the DHCP server
making the offer.

7. DHCP requests
• Whenever a computer comes on line, it checks to see if
it currently has an IP address leased. If it does not, it
requests a lease from a DHCP server.
• Because the client computer does not know the address
of a DHCP server, it uses 0.0.0.0 as its own IP address
and 255.255.255.255 as the destination address. Doing
so allows the client to broadcast a DHCPDISCOVER
message across the network.
• Such a message consists of the client computer's Media
Access Control (MAC) address (the hardware address
built into the network card) and its NetBIOS name.

8. DHCP acknowledgement
• When the DHCP server receives the
DHCPREQUEST message from the client, it
initiates the final phase of the configuration
process.
• This acknowledgement phase involves sending
a DHCPACK packet to the client. This packet
includes the lease duration and any other
configuration information that the client might
have requested.
• At this point, the TCP/IP configuration process is
complete.

9. DHCP terminology
• Scope - A scope is the full consecutive range of possible
IP addresses for a network. Scopes typically define a
single physical subnet on your network to which DHCP
services are offered.
• Superscope - A superscope is an administrative
grouping of scopes that can be used to support multiple
logical IP subnets on the same physical subnet.
Superscopes only contain a list of member scopes or
child scopes that can be activated together.
Superscopes are not used to configure other details
about scope usage. For configuring most properties
used within a superscope, you need to configure
member scope properties individually.

10. • Exclusion range - is a limited sequence of IP addresses
within a scope, excluded from DHCP service offerings.
Exclusion ranges assure that any addresses in these
ranges are not offered by the server to DHCP clients in
the network.
• Address pool - After you define a DHCP scope and
apply exclusion ranges, the remaining addresses form
the available address pool within the scope. Pooled
addresses are eligible for dynamic assignment by the
server to DHCP clients on your network.
• Lease - is a length of time that a DHCP server specifies,
during which a client computer can use an assigned IP
address. When a lease is made to a client, the lease is
active. Before the lease expires, the client typically
needs to renew its address lease assignment with the
server. A lease becomes inactive when it expires or is
deleted at the server.

11. • Reservation -You use a reservation to create a
permanent address lease assignment by the
DHCP server. Reservations assure that a
specified hardware device on the subnet can
always use the same IP address. This is done
by mapping the MAC address to an IP.
• Option types - are other client configuration
parameters a DHCP server can assign when
serving leases to DHCP clients. For example,
some commonly used options include IP
addresses for default gateways (routers) and
DNS servers. Typically, these option types are
enabled and configured for each scope.

12. Exercise
1. In which range should a server’s IP lie?
reserved, excluded or address pool
range?
2. A manager wants to have the same IP
whenever her PC starts. In which range
would her IP lie?
3. There are 10 PCs in a lab used by
students. In which range would their IPs
lie?