Cisco 3750:
IOS Version: 12.2(55)SE8

Yes, I know there are some pricey software tools that will do this with a couple clicks. But I don’t have budget for them. So here is a simple way to determine the IP address of a device connected to a particular port on a Cisco switch.

First, determine the MAC address:

show mac address-table

This will output the VLAN, MAC address and the port for all the connected devices. Here is an example:

30 000c.2928.8da7 DYNAMIC Gi1/0/18
30 000e.0c30.e42a DYNAMIC Gi1/0/17
30 0018.1900.19e0 DYNAMIC Gi1/0/17
100 000c.29b0.0b89 DYNAMIC Gi2/0/36
100 0080.a399.90f9 DYNAMIC Gi3/0/7
200 0023.246f.c77a DYNAMIC Gi2/0/42
200 2c44.fd28.d315 DYNAMIC Gi2/0/42
200 7427.ea1b.551d DYNAMIC Gi3/0/42
300 0004.006f.47ee DYNAMIC Gi3/0/47
300 0021.b7ae.3f00 DYNAMIC Gi6/0/45
300 0030.c1ad.995b DYNAMIC Gi3/0/48
300 0060.b05f.b290 DYNAMIC Gi4/0/45
300 00c0.ee19.f178 DYNAMIC Gi2/0/48
300 00c0.ee3a.a3c8 DYNAMIC Gi3/0/43

So, if you know the port you can obviously find the corresponding MAC address. Once you know the MAC address, you can determine the IP address by cross referencing that MAC address to the output from a “show arp” command.

Here is some sample output:

Protocol Address Age (min) Hardware Addr Type Interface
Internet 0 000e.0c09.fc6a ARPA Vlan400
Internet – 000a.b8c0.0bc3 ARPA Vlan300
Internet 0 0004.23b5.525c ARPA Vlan400
Internet 0 000e.0c08.b9d4 ARPA Vlan400
Internet 0 000c.2934.423f ARPA Vlan400
Internet 4 0007.e924.54cc ARPA Vlan400
Internet 0 000e.0c08.f4e4 ARPA Vlan400
Internet 2 000e.0c32.350e ARPA Vlan400
Internet 0 0007.e924.1c07 ARPA Vlan400
Internet 0 aa00.0400.0404 ARPA Vlan400

Tacking on an “include” to filter the output, makes it pretty simple:

#show mac address-table | include Gi3/0/45
300 00c0.eeb1.0b57 DYNAMIC Gi3/0/45
#show arp | include 00c0.eeb1.0b57
Internet 2 00c0.eeb1.0b57 ARPA Vlan300

Now, I know that is on switch 3 port 45.