The ARP table on a Cisco device is a list of learned IP address and what MAC addresses they resolve to, this is required as generally switches work at layer 2 with MAC addresses not IP Address’s.

Problem:

If you replace a device with another device that is still using the same IP address as the original device but has a different MAC address you find that you are unable to connect to it due to the ARP record on your Cisco device is still pointing to the original devices MAC address.

If like me you what to see the output of a dubug command via a telnet connection, follow the below steps

1) Enable Virtual Terminal Logging

Telnet into the router and get yourself into privileged (Enable) mode and enter the command

terminal monitor

2) Enable Debugging on Virtual Terminal

The Problem:
I recently had a situation where hosts on my internal LAN needed to communicate with a servers on our DMZ using its public IP address however the packets where being sent to the outside world and where being lost.

So Hosts on the LAN where unable to communicate with servers on the DMZ using their public IP address’s.

Overview of the OSI Model

For Host-to-Host communications to to take place a consistent model is required especially when the Host-to-Host communications is taking place between different vendors. The OSI model was created so different vendors would have a standard way to get data from application to the physical wire.

The OSI model consists of seven layers or steps that data should go though while getting from an application to the physical wire.