We present a software tool to find out which devices connected to the local network lower the Internet speed.

It often happens that you find yourself with an extremely slow network connection, in your company, in your practice or at home. In many cases you rush to blame the provider of the Internet connection (and in many cases this is true), but much more often the problem lies in the “immoderate” use of the network bandwidth by devices connected to the local network.

As we have often documented, the download speed is closely linked to the upload activities carried out from the devices connected to the local network. Even just one device connected to the LAN needs to perform a heavy data upload occupying all the bandwidth available in upstream to reduce the download and upload performance of all the other systems connected to the modem router via Ethernet or WiFi cable.

In the article Internet Speed, how to check it and what to do if the contract is not respected, we saw how to test the speed of your Internet connection without making mistakes.

The connection speed test, always if carried out according to the suggestions described in the article, will exclude problems attributable to the connectivity provider.

Once it has been ascertained that one or more devices connected to the local network are overly occupying the bandwidth available in downstream and/or upstream, how can we verify which one is involved?

In the articles Limiting the bandwidth used by programs and devices connected to the local network and Monitoring network traffic in the LAN and the bandwidth occupied we have provided some practical tips.

In particular, the use of QoS (Quality of Service) policies on the router or at the network switch level and the possible setting of VLANs (VLANs: what they are, how to use them and why) make it possible not to lose control over network traffic, giving maximum priority to data transfers carried out by devices that must always enjoy a fast connection.

Many modem routers integrate, in their administration interfaces, tools that show in a textual way or in graphical form the devices connected to the local network that engage more bandwidth in downstream and upstream.

Third-party firmware such as DD-WRT, OpenWRT and Tomato Display Similar Information

Some routers, however, do not provide any tool to match the situation and do not allow you to find out, at a glance, who is drowning the Internet speed.

To test the bandwidth occupied by LAN-connected devices and find out which device is making important data transfers, we suggest you apply the following procedure:

1) Download and install the Virtualbox virtualization software.

2) Create a virtual machine Windows following one of the many guides that we published on IlSoftware.it.

3) After starting the virtual machine and installing the Guest Additions of Virtualbox, go to the Devices, Network, Network Settings menu then choose Card with bridge from the Connect to drop-down menu and Allow everything from the promiscuous mode below (within the Advanced section).

4) Download and install the free software Capsa. Capsa allows you to monitor up to 10 private IP addresses, corresponding to as many devices connected to the local network. The paid versions of the program do not have this limitation (here the comparison between the various versions).

5) When Capsa starts, grant it administrator rights and close the window Thank you for using Colasoft Capsa Free.

6) Leave the Local area network (LAN) connection checked and then click on Set Network Profile and indicate the network bandwidth actually available downstream for the connection in use (in our case we have specified 18 Mbps).

7) After choosing Full analysis and clicking Start, Capsa will start to show what is happening on the previously selected network interface.

Capsa is a packet sniffer: the program is therefore able to identify not only which devices connected to the LAN are generating the data traffic but also the type of each transfer in addition to the source and destination.

The Utilization indicator in the toolbar gives an idea of the percentage of total downstream bandwidth occupied. Try, for example, to measure speed with the speed test fast.com to see the “lancet” go to full scale.

8) If the network connection is slow because it is likely to be affected by some existing data transfer, we suggest that you click on the Capsa MAC Endpoint tab and first click on the bps column: this will give you, at the top of the list, the names of the devices connected to the local network that are transferring more data. For each device is reported the speed instantly detected.

By choosing 1 second from the Refresh button and checking the updated values in the bps and bytes received columns, you will be able to identify the devices that occupy the most downstream bandwidth.

To find out which devices are instantly occupying the upstream bandwidth, we suggest ordering the list by clicking on the Sent/received bytes column.

When the value in bytes shown in this column is very high, it means that the corresponding device is engaged in a particularly heavy data loading.

The PRTG software uses a different and basic approach using SNMP (Simple Network Management Protocol) to obtain real-time information from the router or other devices on the traffic managed. Obviously, to monitor the traffic on the LAN using SNMP it is essential that the network devices support this protocol.