Port forwarding
A rule that lets outside traffic reach a specific device behind NAT, and the reason some proxies can accept inbound connections and others cannot.
Port forwarding is how you punch a deliberate hole through the barrier NAT puts up. Because network address translation hides many devices behind one public address, unsolicited inbound traffic has nowhere to go by default. A port-forwarding rule fixes a mapping, traffic arriving at this public port goes to this internal device and port, so a specific service becomes reachable from outside while everything else stays hidden.
The reason it belongs in a proxy glossary is that it explains an asymmetry people run into. Making an outbound connection through a proxy needs no forwarding, which is why home and mobile connections use proxies without any special setup. Being reachable, running something others connect to, is the case that requires a forwarded port or a public address, and it is exactly what a connection behind carrier-grade NAT cannot provide, since there is no unique public address to forward from.
That constraint shapes what different supply can do. A datacenter or dedicated address with its own public IP can accept inbound connections and host a listening service. A residential exit borrowed from a household behind ordinary NAT, and especially one behind carrier-grade NAT, is typically outbound-only, which is fine for browsing and scraping and rules out anything that needs to be dialled into. When a provider markets inbound-capable residential, this is the property they are claiming to have solved.
For self-hosting a proxy the same rule is the setup step people miss. Standing up your own proxy on a home connection means forwarding its port on the router, and then discovering whether the ISP has placed you behind carrier-grade NAT, in which case forwarding on your own router is not enough because the address you would forward to is itself shared upstream. That discovery is the usual wall between wanting to run a proxy at home and being able to reach it.
Frequently asked questions
Do I need port forwarding to use a proxy?
Not to use one for ordinary browsing or scraping. Outbound connections to a proxy pass through NAT without any special setup, which is why every home and mobile connection can use proxies. Port forwarding is only needed when something must be reachable from outside, such as a proxy or service you are hosting yourself for others to connect to.
Why can't I port forward on my connection?
Most often because you are behind carrier-grade NAT. When the provider shares one public address across many subscribers, forwarding a port on your own router is not enough, since the address you would expose is itself shared upstream and not uniquely yours. A static IP or a business line removes the extra layer; on standard consumer plans it frequently cannot be done at all.
Why are some residential proxies outbound-only?
Because the exits sit behind ordinary or carrier-grade NAT with no forwarded port, so nothing outside can address them directly. They can open connections out perfectly well, which covers browsing and scraping, but cannot accept connections in. Inbound-capable residential requires solving exactly this reachability problem, which is why it is a distinct and rarer offering.
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