CCNA Networking · Quick Reference

Module 9 — Address Resolution Cheat Sheet

CCNA Networking

In one line: Two different address types work together to deliver data across a network:

Key Ideas

1Why Both IP and MAC?. Two different address types work together to deliver data across a network:

2ARP Overview. ARP (Address Resolution Protocol, RFC 826) maps a known IPv4 address to an unknown MAC address on the local network. When a host needs to send a frame but only knows t...

3ARP Operation — Request and Reply. ARP works in two steps: a broadcast request asking "who has this IP?" followed by a unicast reply from the device that owns that IP:

4ARP Issues. ARP has two significant problems in production networks:

5IPv6 Neighbor Discovery (NDP). IPv6 does not use ARP. Instead it uses the Neighbor Discovery Protocol (NDP), which is part of ICMPv6 (Internet Control Message Protocol version 6). NDP is more effici...

Code Examples

! PC1 wants to ping 192.168.1.1 (gateway)
! PC1's ARP cache is empty initially
C:\> arp -a
Interface: 192.168.1.10
  Internet Address   Physical Address   Type
  (empty)

! PC1 broadcasts ARP request: "Who has 192.168.1.1?"
! Router replies: "I d...

! Check ARP table on switch
SW1# show arp
Protocol  Address      Age(min)  Hardware Addr    Type  Interface
Internet  192.168.1.1  0         0019.e81a.b700   ARPA  Vlan1
Internet  192.168.1.10 5         0050.56b1.2345   ARPA  Vlan1

! Clear ARP cache ...

! View IPv6 neighbor table (equivalent of IPv4 ARP table)
R1# show ipv6 neighbors
IPv6 Address                              Age Link-layer Addr State   Interface
FE80::1A2B:3C4D:5E6F:7890               0   0050.56b1.1234  REACH   Gig0/0
2001:DB8::1     ...