How to know the ip address of the device connected to a physical interface

Last updated: Dec 4, 2021

We will see here how to make the correspondence between the ip address of a device and the physical interface of a cisco switch.

The handling will be done in two steps :
- First we will make the correspondence between the mac address and the physical port thanks to the mac table
- We will use the arp table to make the correspondence of the mac address with the ip address.

Prerequisites : The switch must have ip address assigned to the same vlan as the devices we want to know about. I will also detail this step in the example below.

Network Architecture

Let's say we have this very simple architecture and that we want to retrieve the ip addresses connected to Gi1/0/1 and Gi1/0/2 interfaces.

Map addresses ip to physical ports

Set ip address to switch

Addresses needs to be in the same vlan as the devices we want to map.

Configure vlan 1 interface :

switch01(config)# interface vlan 1
switch01(config-if)# ip address 192.168.1.1 255.255.255.0
switch01(config-if)# no shutdown

Get mac addresses

As explained before (switches are Layer 2 devices), there is no way to retrieve directly this information, we first need to get the mac addresses.

So let's see how to retrieve mac addresses connected to this two physical ports.

Get Gi1/0/1 mac address :

switch01# show mac address-table | include Gi1/0/1
   1    24b6.fd14.0853    DYNAMIC     Gi1/0/1

Get Gi1/0/2 mac address :

switch01# show mac address-table | include Gi1/0/2
   1    f8db.8845.ef15    DYNAMIC     Gi1/0/2

Note the informations retrieved :

Interface	mac address	ip address
Gi1/0/1	24b6.fd14.0853	-
Gi1/0/2	f8db.8845.ef15	-

Get ip addresses

Ping ip addresses you want to retrieve in order to populate arp table :

switch01# ping 192.168.1.200
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.1.200, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 36/51/74 ms

switch01# ping 192.168.1.210
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.1.210, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 33/48/71 ms

Show arp table to match ip address with mac address :

switch01# show arp | include 24b6.fd14.0853
Internet  192.168.1.210              0   24b6.fd14.0853  ARPA   Vlan1

switch01# show arp | include f8db.8845.ef15
Internet  192.168.1.200              0   f8db.8845.ef15  ARPA   Vlan1

We can now deduce from this information that 192.168.1.210 device is connected to Gi1/0/1 and 192.168.1.200 device is connected to Gi1/0/2.

Interface	mac address	ip address
Gi1/0/1	24b6.fd14.0853	192.168.1.210
Gi1/0/2	f8db.8845.ef15	192.168.1.200

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