A "closed" port and a "filtered" port look different to a port scanner — and that difference is exactly how NAT and home routers create the illusion of a firewall without anyone configuring one
Port scanning results typically fall into three categories: open, closed, and filtered. For home networks, understanding why most ports appear "filtered" from the internet — not because of any firewall rule someone wrote, but as a side effect of how NAT (Network Address Translation) fundamentally works — explains both why home networks have a degree of built-in protection and why "port forwarding" is necessary for certain applications.
Open, closed, and filtered: what each result means
Open: something is actively listening on this port and accepted the connection attempt (for TCP) or responded (for protocols where this is meaningful). A web server listening on port 443 would show as "open" if scanned.
Closed: the host is reachable, and responded — but specifically with "nothing is listening on this port" (for TCP, this is typically a RST/reset packet in response to a SYN). The host exists and is responding to network traffic in general; this specific port simply has no listener.
Filtered: no response was received at all — the scanner can't determine whether the port is open or closed, because something (a firewall, router, or other network device) is silently dropping the packets without any response. From the scanner's perspective, "filtered" looks the same whether the destination doesn't exist, is down, or exists but has a firewall silently dropping everything to that port.
How NAT creates "filtered" without any explicit firewall rule
The fundamental NAT problem for incoming connections: a home network typically has one public IP address (assigned by the ISP to the router), shared among multiple devices on the private network (each with private IP addresses like 192.168.1.x). When a device on the private network initiates a connection to the internet (e.g., your laptop connects to a website), the router's NAT mechanism creates a temporary mapping: "outgoing traffic from 192.168.1.50:54321 to 93.184.216.34:443 corresponds to this NAT session" — and replies to that connection (the website's responses) are correctly routed back to 192.168.1.50 based on this mapping.
For unsolicited incoming connections (someone on the internet trying to connect to your home network's public IP on some port, without any device on your network having initiated anything): there's no NAT mapping for this — the router has no record of "this incoming connection on port X corresponds to which internal device." With no mapping to use, the router has nothing to do with this incoming packet except drop it.
This is "filtered" behaviour — but it's not because anyone configured a firewall rule saying "block incoming connections." It's an inherent consequence of how NAT works: incoming connections that don't correspond to an existing NAT mapping have nowhere to go, so they're dropped. The effect is similar to a firewall (unsolicited incoming connections don't reach internal devices), but the mechanism is different — it's a side effect of address translation, not a deliberate security policy (though the security benefit is real and is one reason NAT is sometimes informally described as providing a degree of protection, even though that wasn't NAT's original design purpose, which was primarily to address IPv4 address exhaustion by allowing many devices to share one public IP).
Port forwarding: explicitly creating the mapping NAT doesn't have by default
Port forwarding is a router configuration that explicitly creates a persistent mapping: "incoming connections to the router's public IP on port X should be forwarded to internal device Y's IP address on port Z" — regardless of whether any outgoing connection initiated this (unlike the automatic, temporary NAT mappings created by outgoing connections).
Common use cases for port forwarding:
- Running a game server, web server, or other service on a home network that needs to accept incoming connections from the internet
- Remote access to a home device (e.g., a security camera system, a home media server) from outside the home network
- Certain peer-to-peer applications that work better (faster connections, more reliable behaviour) when incoming connections can reach a specific device directly, rather than relying solely on outgoing-connection-initiated NAT traversal techniques
The security trade-off: port forwarding explicitly creates a path for unsolicited incoming connections to reach a specific internal device on a specific port — which is necessary for the use cases above, but also means that device+port combination is now reachable from the internet in a way it wasn't before, with whatever security implications that has for the specific service running there (it should be a service intended to be internet-facing, kept updated, and ideally with its own access controls — port forwarding bypasses the "NAT happens to provide some protection" effect specifically for the forwarded port).
UPnP: automatic port forwarding, and its security implications
UPnP (Universal Plug and Play) includes a mechanism (Internet Gateway Device Protocol) by which devices on the local network can request the router to create port forwarding mappings automatically, without manual configuration by the user — many games consoles, some applications, and various IoT devices use this to enable incoming-connection-requiring features without requiring users to manually configure port forwarding.
The security consideration: UPnP means that any device on your local network (including potentially a compromised device, or malicious software running on an otherwise-trusted device) can request the router to open ports for incoming connections — without the user explicitly approving each such request (UPnP requests are typically handled automatically by the router, by design, since requiring manual approval for every UPnP request would defeat its "automatic, no configuration needed" purpose). This is part of why UPnP is sometimes recommended to be disabled in security-focused configurations, particularly on networks with IoT devices of uncertain security posture — at the cost of needing manual port forwarding configuration for any application that would otherwise have used UPnP.
Why scanning your own home network from outside shows mostly "filtered," and what "open" would mean
If you scan your home network's public IP from an external vantage point (e.g., using an online port scanner, or a cloud server you control), the typical result is that most or all ports show as "filtered" — consistent with the NAT-without-explicit-mapping behaviour described above.
Any ports showing as "open" would indicate either:
- A port-forwarding rule has been configured, directing that port to some internal device's listening service
- A UPnP-created mapping (potentially without the user being aware, depending on what devices/applications on the network have made UPnP requests)
- In less common home setups, the router itself might have a management interface listening on a port that's reachable from the internet (generally not recommended/default for consumer routers, but configuration varies)
For most home users, "everything shows filtered from outside" is the expected and generally desirable baseline — representing the default NAT behaviour without any specific incoming-connection paths having been opened.
How to use the Port Scanner on sadiqbd.com
- Understand your results in context: "filtered" on a home network's public IP is the expected default — it indicates NAT is doing what NAT does, not necessarily that a firewall has been specially configured
- If checking whether a port-forwarding configuration is working: scan from outside your network (the tool runs from sadiqbd.com's servers, providing an external vantage point) — an "open" result for the forwarded port (assuming the internal service is running and listening) indicates the forwarding is correctly reaching the internal device
- For servers (not home networks) — covered in previous articles — "filtered" might indicate a security group or firewall is blocking the port, which may or may not be intentional depending on what that port is supposed to be used for
Frequently Asked Questions
If NAT provides some protection automatically, do I still need a firewall on my home network? NAT's "protection" is a side effect of address translation for unsolicited incoming connections without an existing mapping — it doesn't protect against threats that don't rely on unsolicited incoming connections (e.g., malware on a device that initiates outgoing connections to a malicious server, phishing, compromised software updates, and many other threat categories) — so "NAT provides some incidental protection for one specific category of network-level threat" isn't equivalent to "a comprehensive security posture," and most security guidance continues to recommend additional measures (keeping devices updated, using endpoint security software, safe browsing practices, etc.) regardless of NAT's incidental effects.
Why does port forwarding sometimes not work even after configuring it correctly on the router? Some ISPs use "carrier-grade NAT" (CGNAT) — where the IP address your router considers its "public" IP is actually itself a private address from the ISP's perspective, with the ISP performing an additional layer of NAT to a truly public IP shared among multiple customers. In CGNAT setups, port forwarding configured on your router only affects the NAT layer your router controls — it can't create mappings at the ISP's NAT layer, which is outside your control entirely. This is a genuinely unresolvable limitation from the customer's side (short of requesting a different service tier from the ISP that provides a true public IP, where offered) and a common reason "I configured port forwarding correctly but it still doesn't work" occurs.
Is the Port Scanner free? Yes — completely free, no sign-up required.
Try the Port Scanner free at sadiqbd.com — check open, closed, and filtered ports on any host or IP address.
