NAT Type Selector — TechKnowSurge

About this interactive

Every NAT type does the same basic job — rewrite an address on the way past the router — and they differ only in three things: how many inside hosts share how many public addresses, where those public addresses come from, and whether port numbers are part of the mapping. Get those three questions into your head and the four types sort themselves. Static NAT is one inside address bolted permanently to one public address. It is what you configure when something outside needs to find something inside at a predictable place — a web server, a camera, a VoIP system registering an SIP trunk. The tell in a scenario is the word always paired with a single fixed address: the mapping does not expire and does not move. Dynamic NAT is also one-to-one, but the public address is handed out from a NAT pool on demand and returned when the session ends. Because the pool is finite, Dynamic NAT can run out — and that is its signature. Any scenario mentioning a pool, a block, first-come-first-served, or connections failing at peak hours is describing Dynamic NAT, because only Dynamic NAT has a supply that can be exhausted. Twenty users and ten public addresses is not a misconfiguration; it is the design, and the eleventh simultaneous user waits. PAT, also called NAT overload, breaks the one-to-one rule entirely. Many inside hosts share a single public address, and the router keeps them apart by rewriting the source port as well as the address, so each conversation gets a unique port on the outside. This is why fifty devices behind one home router all reach the internet at once on one public IP, and it is why PAT is the NAT you actually meet: IPv4 addresses are scarce, and PAT is the mechanism that lets an ISP hand a household or a small business exactly one and have it be enough. If a scenario has many devices and one public address, it is PAT. Static PAT is the corner that catches people, because it borrows from both sides. Like Static NAT it is permanent and configured by hand; like PAT it involves ports. The difference is direction and specificity: Static PAT is a standing rule that inbound traffic arriving on one public port goes to one internal host on one internal port — external 80 to 192.168.1.10:80, external 2222 to an internal SSH server, UDP 27015 to a game server. You may know it as port forwarding, which is the same thing under a friendlier name. The discriminator against plain Static NAT is whether a port number appears in the mapping; the discriminator against plain PAT is whether the rule is permanent and inbound rather than created on the fly by outbound traffic. When you are stuck on an item, ask the three questions in order — how many to how many, from a pool or fixed, ports in the mapping or not — and each scenario lands in exactly one bin.

What you'll learn

Classify NAT scenarios by type: Static NAT, Dynamic NAT, PAT, or Static PAT
Identify the defining characteristic of each NAT type: one-to-one permanent, one-to-one from a pool, many-to-one with port translation, or a permanent port-mapped entry
Recognize why PAT is the most common form of NAT in home and small business networks

Aligned to

Cisco CCNA

4.1 Configure and verify inside source NAT using static and pools

CompTIA Network+

2.1 Explain characteristics of routing technologies.

Key terms

Network Address Translation: A method that maps private IP addresses to a public IP address, allowing multiple devices to share a single public IP.
Static NAT: A one-to-one mapping between a single inside local IP address and a single inside global IP address, configured with the ip nat inside source static command.
Dynamic NAT: A NAT configuration that uses an ACL to identify inside local addresses and a NAT pool to supply the global addresses they are translated to.
Port Address Translation: A form of NAT that maps multiple devices on a private network to a single public IP address by differentiating each connection using unique source port numbers. PAT is the most common NAT implementation and is what enables an entire home or office network to share a single public IP address.
Static NAT Port Forwarding: A permanent, one-to-one mapping on a router that directs inbound traffic arriving on a specific public IP and port to a designated internal IP address and port.
NAT Pool: A named range of public IP addresses configured on a Cisco router that dynamic NAT or PAT draws from when translating inside local addresses to global addresses.
Inside Local: The private IP address assigned to an inside host as seen from within the local network, found in the IP header before translation.
Inside Global: The public IP address that represents an inside host to the outside world, found in the IP header after translation.
Outside Global: The IP address of an outside destination host as seen from the public network, found in the IP header after translation.
NAT Table: A translation table maintained by a NAT-enabled router that records mappings between inside local and inside global addresses to ensure return traffic is correctly forwarded.

Topics

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