Cisco ICND1 Flashcard: DHCP

The starting point for understanding DHCP for the CCNA Exam or ICND1 and ICND2, is that DHCP is based on a client-server model.

There are three types of IP address allocation by DHCP

1. Automatic - assigns a permanently binding IP address to the client
2. Dynamic - assigns an IP address to the client for a limited time, until lease expires and the IP address is returned to the pool for reuse
3. Manual - a network administrator assigns the IP address to the client, DHCP conveys it (not completely sure I understand this to be a kind of DHCP).

The Client requests an IP address allocation and initialization parameters from the DHCP Server through the following exchange of messages.

DHCPDISCOVER
When a client boots up for the first time, it sends a DHCPDISCOVER message broadcast to 255.255.255.255 all destinations with a source IP address of 0.0.0.0 (because it doesn't have one)

DHCPOFFER
The DHCP server receives the message and replies with a DHCPOFFER unicast, and contains fields to specify a subnet mask or the default gateway (router), and other values including the IP address lease time, renewal time, DNS, and NetBIOS name. The message is sent on Layer 2 to the client MAC address. The destination IP address is the address being offered by the server.

DHCPREQUEST
Client responds to the offer with this message, showing intent to accept the parameters, sent to broadcast address (Layer 2 and Layer 3), uncertain if address is safe to use or if another DHCP client will grab it.

DHCPACK
The DHCP Server receives the request message, acknowledges the request with this unicast message

Setting DHCP Pool Parameters using a Wizard
The DHCP pool may be configured from the Cisco SDM page, according to the parameters below
DHCP Pool Name - Name that clearly identifies the DHCP pool
DHCP Pool Network and Subnet Mask- the assigned IP address is drawn from the pool, specifying a starting and ending IP address in the range.

Hints: The IP address range should be within the private address range
10.1.1.1 to 10..255.255.255
172.16.1.1 to 172.31.255.255
192.168.0.0 to 192.168.255.255

Must not use the reserved address in this range specified
The network or subnetwork IP address
The broadcast address on the network

Starting IP
Ending IP
Lease Length - integer number in days
DHCP options
DNS Server 1
DNS Server 2
Domain Name
WINS Server 1
WINS Server 2
Default Router - IP address of router to use as the default gateway
Import all DHCP options into the DHCP Server database

CLI Cisco IOS DCHP server on a router
ip dhcp pool mydhcppool
network 10.10.10.0 /8
domain-name mydhcpdomain.com
dns-server 10.10.10.98 10.10.10.99
default-router 10.10.10.1
lease 7
exit
ip dhcp excluded-address 10.10.10.0 10.10.10.99