Network address translation and WAN IP configuration
Network address translation and WAN IP configuration
Good evening! I'm a bit puzzled about NAT. The confusion comes from the IP addresses involved, like static NAT where we manually link private IPs to public IPs and ports. Does any IP used in NAT relate to the WAN IP provided by your ISP? Thank you for your time!
NAT usually isn't related to the IP your ISP gives you. My ISP Comcast gives IPs based on the MAC address of the device connected to the modem. Typically, home networks get IPs from a built-in DHCP server in the router. These private IPs, like 192.168.x.x or 10.x.x.x, aren't internet addresses and can't be used online—they're just for routing traffic within your network. I believe you're thinking about the firewall; that's where you set up ports to forward. In this scenario, you'd assign a static IP to the device outside the DHCP range. The main purpose of NAT is letting you share one public IP across several machines.
The process involves a direct one-to-one conversion between internal and external IP addresses. When you use NAT, each internal device gets a unique external IP, but if you have only one public IP, it switches to PAT (Port Address Translation). The router handles this by mapping ports to IP addresses instead of doing a full IP-to-IP translation, allowing traffic to be routed correctly back in.
The router includes a component for handling this. Typically, it assigns a single external IP from your ISP. Traffic flows through the public IP within certain local ranges, such as 192.C, 172.B or 10.A. Each local IP can have its ports forwarded to the public IP, like Lurick mentioned. But if you open ports for HTTP(S) like 80 and 443, assigning the same port ranges to another local IP would create conflicts in the NAT regarding packet routing. The NAT enables using a range of local IP addresses instead of being restricted to just one ISP-provided address. That’s why it’s called Network Address Translation. Your question should clarify: Will every port forwarded IP always face the external network? Yes, they will.
In short, the Internet only sees your WAN IP while your LAN understands it as a range. The router acts as an intermediary, translating between your local network and the outside world. NAT swaps the WAN IP with your LAN IP when sending data, allowing communication across networks. Port forwarding helps the router recognize incoming traffic from the WAN as coming from its LAN address. Ultimately, the router manages these transitions, remembering which IP to send back to the Internet.
Thank you for your feedback. I appreciate the detailed insights you shared. I understand now that my explanation may have been unclear. Regarding your questions, at static or dynamic NAT, the public IP address the router uses is typically the one provided by the ISP. If it differs from the ISP's WAN IP, NAT determines whether the desired internal IP is already in use by another device on the network. The criteria for assigning an internal public IP involve checking availability and ensuring no conflicts exist. Please let me know if you'd like further clarification!
The router receives the ISP's WAN IP address. It doesn't have any information about other devices using it, but your ISP already knows this from their records. Assigning a static WAN IP is possible, especially if you receive several options, but you must ensure you don’t reuse one. If you do, issues will arise. With multiple WAN addresses, your ISP provides a subnet, designating one IP as a network address and another as a gateway for communication with the wider Internet. Your router uses this to send data to the correct subnet, just like your own private IPs. Public IPs are meant to belong to the broader Internet, not your local network. You can use public IP ranges in two ways: multi-NAT, where you specify which IP to route traffic through, or a DMZ, placing clients on the public side and assigning those IPs. In a DMZ, the router acts like any other device on the Internet, forwarding traffic without NAT. From your local network, devices appear normal, but the router must handle NAT if needed. One of your public IPs still needs to be linked to the router for proper operation.
LAN IP and WAN IP operate separately. Your local router monitors external connections and assigns a specific port from the ISP-provided WAN IP. The ISP has no knowledge of your LAN configuration; they only see a single device. When traffic returns on an allocated port during a session, the router checks its table to determine where to forward the data internally. Using the session table, it maintains mappings between ports and LAN clients, preventing conflicts. These entries have a lifespan determined by the router and settings—some systems like Cisco IOS retain a PAT entry for up to 24 hours unless a TCP RST is detected, which then updates the NAT entry to a shorter duration.