Port 68: DHCP client
Port 68 is where devices listen for replies from a DHCP server when they ask for an IP address.
What runs on port 68
DHCP clients send requests from UDP 68 and receive offers and acknowledgements back on it. Every device that gets its address automatically uses it briefly when it joins a network and when it renews its lease.
On a home network
Every phone, laptop, TV and smart device uses 68 as a DHCP client. Your router uses it on its WAN side to get a public address from your ISP.
If port 68 is reachable from the internet
Seeing 68 in a scan simply means a device is a DHCP client. It is not a service anyone can log in to, and it carries no meaningful exposure risk on its own.
How to check whether it is open on your network
There is nothing to close here. If a device is not getting an address, check the DHCP server on your router instead.
To find out whether port 68 is open on something in your home, and whether the internet can reach it, the app scans your Wi-Fi from your phone and checks your public address from the outside. It says what it checked and what it could not. Get RECON for iPhone — one-time purchase, no account, nothing leaves the device.