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content/2.general/1.networking/1.nat.md

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+---
+navigation: true
+title: NAT & DHCP
+main:
+  fluid: false
+---
+:ellipsis{left=0px width=40rem top=10rem blur=140px}
+
+# Router and NAT
+
+::alert{type="info"}
+🎯  __Goals:__
+- Understand how port forwarding works
+- Learn how to configure router NAT
+- Learn how to issue DHCP leases (fixed IPs)
+::
+
+![picture](/img/global/nat.svg)
+
+## What is a "port"?
+---
+Ports are different channels through which your router sends and receives data. This allows multiple services to run simultaneously.  
+When it receives data through a port, your router forwards that data to the machine that:
+- either initiated the request,
+- or is configured to receive data on a specific port.
+
+Your router has over 65,000 ports available.
+
+Some programs and applications are designed to use specific ports. For example, when your network sends data from an HTML page, the router receives it through port 80 (non-secure) or port 443 (secure via SSL).  
+
+So, your router acts as a data dispatcher between the internet and your local machines.
+
+## Port Forwarding
+---
+Forwarding a `port` means setting a rule that specifies which `source` can send data to which `port` on your router, which will then forward it to a specific `port` on a specific `machine`. The `sources` and `destination machine` are identified by their IP addresses.
+
+| Variable               | Description                                             | Example                 |
+|------------------------|---------------------------------------------------------|-------------------------|
+| `source machine`       | IP of the source machine (from the internet)            | `All`<br>`123.45.67.89` |
+| `source port`          | Incoming port on the router                             | `443`                   |
+| `destination port`     | Port on the destination machine                         | `3000`                  |
+| `destination machine`  | IP of the target machine (on your local network)        | `192.168.1.50`          |
+
+According to the table:  
+If we remove `All` and keep the IP `123.45.67.89`, all traffic from this IP sent to port `443` on your router will be forwarded to port `3000` on the local IP `192.168.1.50`.
+
+If we remove the IP and keep `All`, then all traffic from the internet on port `443` will be redirected to port `3000` on `192.168.1.50`.
+
+This is useful when you have a server that must be accessible from the internet. For instance, a website uses port `80` (non-secure) or `443` (SSL-secured).  
+To make the website accessible, you'll configure your router to redirect the domain request to your local server.  
+Assume your service runs on port `3000` locally (`http://192.168.1.50:3000`), you would redirect all traffic from port `443` on the router to port `3000` on the local server.
+
+::alert{type="warning"}
+:::list{type="warning"}
+- __Warning:__ If you have multiple services to expose like `subdomain1.mydomain.com` and `subdomain2.mydomain.com`, your router cannot differentiate requests and forward to different ports.  
+  You must use a [Reverse Proxy](../../serveex/core/swag) to route traffic based on the request.
+:::
+::
+
+## DHCP
+---
+Every time a device connects to your local network, your router assigns it an IP address using DHCP rules.  
+This IP is randomly selected from a predefined pool.  
+At every device reboot, the IP may change — which is problematic if you're forwarding ports, as the target IP may no longer be valid.  
+
+To avoid this, use your router's DHCP server to assign a static IP address.
+
+Each device has a physical "MAC address".  
+To assign a fixed IP, you must know your device's MAC address (visible in your router when it's connected), and assign it a static IP.  
+This is called a "static DHCP lease."
+
+That way, your machine's IP never changes and your port forwarding rules remain effective.
+
+| Variable      | Description                     | Example             |
+|---------------|----------------------------------|---------------------|
+| `IP`          | Fixed local IP to assign         | `192.168.1.50`      |
+| `MAC Address` | Physical address of the device   | `5E:FF:56:A2:AF:15` |
+
+For more information, refer to your router's documentation.