IP, routing and IPv6

MTU problems on a Mac

The MTU is the largest packet that an interface sends. If a link in the path takes only smaller packets, large transfers can stall.

Applies to: macOS 13 Ventura and later for the System Settings steps. The Terminal commands work on all current macOS versions.Checked: 29 October 2026

Symptoms

Some sites start to load, then stop.
Small requests work, but uploads or large downloads stall.
Problems start only with a VPN on, or only on one network.

Likely causes

A link with a smaller MTU. PPPoE, VPN tunnels and some mobile links carry smaller packets than standard Ethernet, which uses 1500 bytes.
Blocked ICMP messages. A firewall that blocks ICMP can stop the Mac from learning that a packet is too large.
A manual MTU setting. A value set by hand can be too large or too small for this network.

How to check

Find the device name of each network port, for example en0 for Wi-Fi.
```
networksetup -listallhardwareports
```
Show the MTU of the interface.
```
networksetup -getMTU en0
```
Send a ping of 1472 data bytes with the Don't Fragment bit set. With 28 header bytes, it makes 1500 bytes.
```
ping -D -s 1472 -c 3 1.1.1.1
```
If no replies come back, try smaller sizes. Find the largest size that gets replies, then add 28.
```
ping -D -s 1464 -c 3 1.1.1.1
```
Show the MTU values that the interface accepts.
```
networksetup -listValidMTURange en0
```

How to fix it

Change the MTU only on the advice of your provider or administrator, or after the test above.
Choose Apple menu > System Settings > Network. Click the service, click Details, then click Hardware.
Set Configure to Manual, then enter the MTU.
Or set it in Terminal. This example uses 1492, a common value for PPPoE.
```
networksetup -setMTU en0 1492
```
Needs an administrator password.
To go back to the automatic MTU, use this command.
```
networksetup -setMTUAndMediaAutomatically en0
```
Needs an administrator password.
For a VPN, change the MTU in the VPN app, if it has this setting.

Questions

Why does 1472 give 1500?

An IPv4 ping packet has a 20-byte IP header and an 8-byte ICMP header. 1472 data bytes plus 28 header bytes make 1500.

Why 1492 for PPPoE?

PPPoE adds its own header inside an Ethernet frame. RFC 2516 limits the PPPoE MRU to 1492 bytes.

Sources

networksetup(8), man page installed with macOS 27.2
ping(8), man page installed with macOS 27.2
Apple Support: Change Hardware settings on Mac
RFC 2516: A Method for Transmitting PPP Over Ethernet (PPPoE)

Sources checked on 29 October 2026. Man pages are the ones installed with macOS; open one with man <name> in Terminal.