Information Technology Reference
In-Depth Information
DHCP and
Internal DNS
Enterprise
Edge
E-Commerce/
DMZ/Internet
DHCP and
Internal DNS
Remote
Modules
Enterprise Campus
Data Center
Enterprise
Branch
External
DNS
Internal and
External DNS
Enterprise
Data Center
Enterprise
WAN
External
DNS
Campus
Core
Building
Distribution
SP Edge
Premise
Enterprise
Teleworkers
Remote Access
VPN
Building
Access
Internet
Figure 8-7
DHCP and DNS Servers in the Network
ARP
When an IP host needs to send an IP packet over an Ethernet network, it needs to find out
what 48-bit MAC physical address to send the frame to. Given the destination IP, ARP ob-
tains the destination MAC. The destination MAC can be a local host or the gateway
router's MAC address if the destination IP is across the routed network. ARP is described
in RFC 826. The local host maintains an ARP table with a list relating IP address to MAC
address.
ARP operates by having the sender broadcast an ARP request. Figure 8-8 shows an exam-
ple of an ARP request and reply. Suppose a router with the IP address 10.1.1.1 has a packet
to send to 10.1.1.10 but does not have the destination MAC address in its ARP table. It
broadcasts an ARP request to all hosts in a subnet. The ARP request contains the sender's
IP and MAC address and the target IP address. All nodes in the broadcast domain receive
the ARP request and process it. The device with the target IP address sends an ARP reply
to the sender with its MAC address information; the ARP reply is a unicast message sent to
10.1.1.1. The sender now has the target MAC address in its ARP cache and sends the frame.