In order to analyze network traffic, check the destination port number using the network monitoring tool that identified the spikes. In many cases, this will tell you the type of TCP and UDP traffic you're seeing. For example, traffic on port 80 is normally HTTP traffic, while traffic on port 443 is normally HTTPS traffic. You can consult the Port Database if you encounter a port you don't recognize.
If that doesn't do the trick, you'll need to sniff the network traffic to identify it. You can do this by connecting a computer running a packet sniffer to your network and leaving it running during one of the spikes. My favorite tool for this job is Wireshark. For more information on using Wireshark, see my tutorial: How to sniff network traffic.
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This was first published in April 2009