DHCP Flashcards
(11 cards)
DHCP - Dynamic Host Configuration Protocol
• DHCP is a client/server protocol that automatically provides a host with its IP address and other related configuration information such as the subnet mask and default gateway.
• DHCP clients obtain their IP configuration information from a DHCP server, rather than being manually configured.
How does Dynamic Host Configuration Protocol work?
DHCP Discover (Broadcast)client
DHCP Offer (Unicast/Broadcast)server
DHCP Request (Broadcast) client
DHCP Ack (Unicast/Broadcast) server
See: DHCP defined by RFC 2131
DHCP Benefits - Reduced Network Admin
• Centralized and automated IP configuration, rather than manually assigning an IP address to every host.
• Can assign additional IP configuration values by means of DHCP options.
• Efficient handling of clients that must be updated frequently, such as laptops that move to different locations on a wireless network.
• The forwarding of initial DHCP messages by using a DHCP relay agent, which eliminates the need for a DHCP server on every subnet.
DHCP Benefits - Reliable IP address configuration
• DHCP minimizes configuration errors caused by manual IP address configuration, such as typos, or address conflicts caused by the assignment of an IP address to more than one computer at the same time.
DHCP Clients
• Desktop PCs are good candidates to be DHCP clients because there will typically be many of them in an office. Using DHCP saves a lot of admin work that would be necessary if manually configuring IP addresses.
• They do not accept incoming connections so it does not matter if their
IP address changes.
• Servers and network infrastructure devices such as routers and switches will not typically be DHCP clients.
• They are mission critical devices which do not move and are required for the network and its services to function.
• Their IP addresses are manually configured to ensure they will not change and are not dependant on DHCP.
Cisco DHCP Server Configuration using Cisco router
R1 (config) #ip dhep excluded-address 10.10.10.1 10.10.10.10
R1 (config) #ip dhep pool 10.10.10.0_ Clients
R1 (dhcp-config) #network 10.10.10.0 255.255.255.0
R1 (dhcp-config) #default-router 10.10.10.1
R1 (dhcp-config) #dns-server 10.10.20.10
• Excluded addresses: 10.10.10.1 to 10.10.10.10 won’t be given to clients. • DHCP pool name: 10.10.10.0_Clients • Network: Clients get IPs from 10.10.10.0/24 (except excluded ones). • Default gateway: 10.10.10.1 • DNS server: 10.10.20.10
To configure a router as a DHCP server, you need to:
- Exclude IP addresses (optional but recommended):
So certain IPs (like the router’s own) aren’t given to clients.
ip dhcp excluded-address <start> <end>
</end></start>- Create a DHCP pool:
ip dhcp pool <pool-name></pool-name> - Set the network and subnet mask:
network <network-address> <subnet-mask></subnet-mask></network-address> - Set the default gateway (router address for clients):
default-router <gateway-IP></gateway-IP> - Set the DNS server (for name resolution):
dns-server <DNS-IP></DNS-IP>
- Create a DHCP pool:
Configuring a Cisco Router as a DHCP Client
• Cisco routers are typically manually configured with static IP addresses
• An exception to this is where an office is connected to the Internet but has not bought static public IP addresses (because it does not contain any publicly available servers which would need a fixed IP address for incoming connections)
• The office still requires a public IP address to allow internal hosts outbound connectivity to the Internet through NAT
• In this case the router will receive the public IP address on its outside interface from the Internet service provider via DHCP
Forwarding dhcp request from router to dhcp server as External DHCP Server Configuration
R1 (config) linterface f0/1
R1 (config-if) “ip helper-addross 10.10.20.10
Configuring a Cisco Router as a DHCP Client commands
R1 (config) #interface f0/0
R1 (config-if) #ip address dhcp
R1 (config-if) #no shutdown
Verification - show dhcp lease
R1#show dhep lease
Temp IP addr: 203.0.113.2 for peer on Interface: FastEthernet0/0
Temp sub net mask: 255.255.255.0
DHCP Lease server: 203.0.113.1 / state: Bound
DHCP Transaction id: 64B8EE07
Lease: 86400 secs, Renewal: 43200 secs, Rebind: 75600 secs
Temp default-gateway addr: 203.0.113.1
Next timer fires after: 11:53:13
Retry count: 0 Client-ID: cisco-0001. 63C2. 9701-Fa0/0
Client-ID hex dump: 636973636F2D303030312E363343322E
93730312D4661302F30
Hostname: R1
