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Home file server behind CGNAT: remote access to your files

Your network storage or home file server works fine at home, but you can't open or sync your files from outside. If your ISP uses CGNAT, outside connections never reach it.

Why CGNAT gets in the way

Opening or syncing files from outside means connecting into your network, to the file server's port. Behind CGNAT, your router doesn't hold a public IPv4 address, so those connections stop at the ISP's NAT, which only the ISP can open (RFC 6269, section 5.2.1).

What works and what doesn't

File sharing and backups inside your home network: unaffected.
Your file server backing up to somewhere else on the internet: that's outgoing, so it works.
Reaching your files from outside over IPv4: blocked, unless your ISP gives you a public IPv4.
Reaching them over IPv6: can work if your ISP gives you IPv6, the network you're on has IPv6 too, and your router's IPv6 firewall allows the port.

Check whether you're behind CGNAT

Compare your router's WAN address with your public IP using the CGNAT checker or the step-by-step guide. A WAN address between 100.64.0.0 and 100.127.255.255 is the CGNAT range (100.64.0.0/10 explained). If the two addresses match, CGNAT isn't your problem; check the port forward and any inbound port blocks your ISP lists.

What to ask your ISP

"Can you move my line off CGNAT and give me my own public IPv4?" Some ISPs do this for free on request; others only with a paid static IP.
"Do you sell a static IP on residential plans, and what does it cost?" Some ISPs only sell it on business plans.
"Is IPv6 available on my line, and is it turned on?" IPv6 doesn't go through the CGNAT, but the other side needs IPv6 too.
"What upload speed does my plan really have?" Files you open from outside leave your house over your upload. The ISP finder shows which providers offer symmetrical upload.

Which ISPs make this easier

Of the 41 ISPs we track, 12 give home customers a public IPv4 by default, 3 offer a free CGNAT opt-out and 8 sell a static IP on home plans, per our sources. See ISPs without CGNAT, ISPs that use CGNAT, or filter the ISPs without CGNAT that offer symmetrical upload in your state. Each ISP page also quotes what the terms say about running servers at home.

FAQ

Why can't I reach my home file server from outside?

If your ISP uses CGNAT, your router doesn't have its own public IPv4 address, so incoming connections from the internet stop at the ISP's NAT and never reach your file server.

Do local backups still work behind CGNAT?

Yes. Traffic between devices on your home network isn't affected, and backups your server sends out to the internet are outgoing connections.

What should I ask my ISP?

Ask whether it can move your line off CGNAT to a public IPv4, whether it sells a static IP on home plans, and whether IPv6 is available and turned on. Check your upload speed too.

This site sticks to ISP facts. It doesn't review or recommend third-party products or services.