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What is MTU?

MTU stands for Maximum Transmission Unit. It’s the largest size a packet can be before it gets sent across a network connection. 

MTU is measured in bytes, and every network has an MTU limit. When data exceeds that limit, it has to be broken into smaller pieces before it can travel. Most standard internet connections use an MTU of 1500 bytes, so that has become a widely accepted default. But not every network along a given path uses the same limit, and that’s where you could encounter a problem. 

If a packet arrives at a network that can’t handle its size, one of two things happens. The first option is that the packet gets fragmented into smaller pieces and reassembled later. The second option is that it simply gets dropped. Dropped packets mean retries, delays, and in some cases, connections that seem to stall for no obvious reason.

For most users on standard home networks, MTU works quietly in the background. It only tends to surface when something isn’t connecting the way it should.

If you’re using a VPN, you may be more likely to experience MTU mismatches. Because a VPN adds extra data to each packet as part of the encryption process, this can push packets over the limit of the underlying network. 

The fix is usually to lower the MTU setting so there is enough room for the necessary overhead.