Networking 101 - A host seems down, the first thing most engineers do is Ping it. Nothing wrong with this step, except when the host replies, but the host is still down? This is perfectly normal in many situations, yet some network engineers really do get stumped how a server can respond to ping, but still not give any shiny web pages. In fact, Ping is used far too often for miss-diagnosing problems, so I thought I’d help my fellow engineers out and remind them what ping actually does. Ping uses ICMP to communicate to a remote host. ICMP is a layer 3 protocol, so a working Ping means working IP. Routing, IP addressing, subnets, VLAN’s all check out OK.

But there are still 4 other layers of the OSI model which could be at fault, in other words - Ping only gets you 3 levels up. In the last situation, a firewall was blocking port 8443, the custom port the web server was running on - the most basic schoolboy error of networking professionals. Ping worked because ICMP was not blocked, however no web pages would obviously get through. It amazed me how Ping was as far as this particular network ‘professional’ went, without consideration for the higher layers (and I remember thinking this ‘OSI model was all crap back in the day!). Bottom line is that Ping is but one tool used to troubleshoot networking issues. It’s perfectly normal to have a service unavailable yet still Ping, and vice-versa. It only troubleshoots up to layer 3, so you’ll need to utilize other skills to troubleshoot the remaining layers. It’s a convenience tool courteously given to us by the good folks who wrote TCP/IP, but it does nothing to help troubleshoot HTTP, CIFS, or SSL issues.

	Posted March 14, 2012
	Written by John Payne