Network Address Translation (NAT)

Summary

This lecture delves into Network Address Translation (NAT), a method used to map multiple local IP addresses to a single public IP address, conserving IP resources. The speaker explains how NAT operates by altering the source IP and port of outgoing datagrams, and maintaining a translation table for incoming datagrams. The lecture also touches on the benefits of NAT, such as reduced need for public IP addresses, ease of local network changes without external notification, and enhanced security by shielding internal IP addresses. It briefly discusses the controversy surrounding NAT due to its layer four processing and the ongoing shift towards IPv6 to address IP address limitations. Finally, the lecture addresses how external clients can connect to servers behind a NAT, either through static configuration or dynamically using protocols like Universal Plug and Play (UPnP).

Takeaways

• 🌐 Network Address Translation (NAT) allows a local network to use private IP addresses and share a single public IP address for external communication.
• 🔄 NAT maps multiple private IP addresses to a single public IP address, facilitating communication between a local network and the internet.
• 🏢 The need for NAT arises from the limited number of public IP addresses available and the desire to conserve these resources.
• 🔒 NAT provides an additional layer of security by making internal network devices inaccessible directly from the internet.
• 🛠️ NAT implementation involves replacing the source IP address and port number of outgoing datagrams with a public IP address and a new port number.
• 📝 The NAT translation table keeps a record of the mappings between private IP addresses and ports to public IP addresses and new ports.
• 🔀 For incoming datagrams, the NAT router consults the translation table to determine the correct internal host and port to forward the data to.
• 🚧 The port field, being 16 bits long, allows for up to 65,536 simultaneous connections with a single public IP address.
• ⚖️ There is controversy over NAT because it involves routers processing information at the transport layer, which is against the traditional layer-based model.
• 🌐 The long-term solution to IP address scarcity is the adoption of IPv6, which offers a vastly larger address space.
• 🔄 Universal Plug and Play (UPnP) can dynamically configure NAT settings, avoiding the need for static configurations and manual updates.

Q & A

What is Network Address Translation (NAT)?

Network Address Translation (NAT) is a method of remapping one IP address space into another by modifying network address information in the IP header of packets while they are in transit across a traffic routing device.

Why is NAT used in local networks?

NAT is used to allow multiple devices in a local network to share a single public IP address. This conserves the number of public IP addresses needed and allows for internal IP addressing to be changed without affecting external communications.

How does NAT provide security?

NAT provides security by making the internal IP addresses of a network inaccessible to the outside world. This is because the internal devices are not directly addressable; they are only reachable through the NAT device.

What is the significance of the NAT IP address?

The NAT IP address is the public-facing IP address that all datagrams from the local network use when they leave the local network. It is the address that the rest of the internet sees.

How does NAT handle incoming datagrams?

NAT handles incoming datagrams by using a translation table to match the public IP and port with the corresponding private IP and port of the internal device.

What is the role of the NAT translation table?

The NAT translation table stores mappings between the private IP addresses and ports of local devices and the public IP address and port used for external communication.

Why is NAT considered controversial?

NAT is controversial because it requires routers to process information up to the transport layer (layer 4), which goes against the traditional model where routers operate only up to the network layer (layer 3).

How does the shortage of IPv4 addresses relate to NAT?

The shortage of IPv4 addresses, which are limited to 2^32 combinations, has led to the widespread use of NAT to allow more devices to share a single public IP address.

What is the Universal Plug and Play (UPnP) protocol in relation to NAT?

UPnP is a protocol that can dynamically configure NAT settings to allow devices behind the NAT to receive incoming connections without manual configuration of port forwarding rules.

What is the difference between static NAT configuration and using UPnP?

Static NAT configuration requires manual setup of port forwarding rules, which can be cumbersome and inflexible. UPnP, on the other hand, dynamically manages these rules, making it easier to add or change devices on the network.

How does a client outside the network connect to a server behind a NAT?

A client outside the network can connect to a server behind a NAT by connecting to the public NAT IP address and the specific port that has been configured to forward to the internal server.