We have one central server which functions as an internet gateway. This server is connected to the internet, and using iptables we forward traffic and share the internet connection among all computers in the network. This works just fine. However, sometimes internet gets really slow. Most likely one of the users is downloading videos or other large files. I want to pinpoint the culprit. I'm thinking of installing a tool that can monitor the network traffic that passes through the server, by IP. Preferably in real time as well as an accumulated total (again by IP). Any tool that is recommended for this? Preferably something in the  Ubuntu repositories. 
Find out network traffic per IP
networking;monitoring;ip;router;bandwidth
I'm going to have to be cheap and copy my answer from this question.ntop is probably the best solution for doing this. It is designed to run long term and capture exactly what youre looking for.It can show you which clients are receiving/sending the most traffic, where theyre recieving/sending to, what protocols and ports are being used etc.It then uses a web GUI to navigate and display this information.ntop is a fairly well known tool, so I would be highly surprised if its not in Ubuntu's package repository.