The following is a summary of each topic in the chapter and some questions for your reflection.
What Did I Learn in this Module? (12.3.1)
• Network Boundaries— Every host on a network must use the router as a gateway to other networks. Therefore, each host must know the IPv4 address of the router interface connected to the network where the host is attached. This address is known as the default gateway address. It can be either statically configured on the host or received dynamically by DHCP.
The wireless router acts as a DHCP server for all local hosts attached to it, either by Ethernet cable or wirelessly. These local hosts are referred to as being located on an internal, or inside, network. When a wireless router is connected to the ISP, it acts like a DHCP client to receive the correct external network IPv4 address for the internet interface. ISPs usually provide an internet-routable address, which enables hosts connected to the wireless router to have access to the internet. The wireless router serves as the boundary between the local internal network and the external internet.
• NAT Operation—The wireless router receives a public address from the ISP, which allows it to send and receive packets on the internet. It, in turn, provides private addresses to local network clients.
The process used to convert private addresses to internet-routable addresses is called NAT. With NAT, a private (local) source IPv4 address is translated to a public (global) address. The process is reversed for incoming packets. The wireless router is able to translate many internal IPv4 addresses to the same public address, by using NAT.
Only packets destined for other networks need to be translated. These packets must pass through the gateway, where the wireless router replaces the private IPv4 address of the source host with its own public IPv4 address.
It turns out that the IPv4 addresses on the devices in my home network are private addresses that are only used in my LAN. But if I need to venture out beyond my home network, perhaps to go to the internet, or send an email to someone outside of my network, my device needs to be assigned a public address. How does your router know if you are trying to get access to a device or a website that is outside of your LAN? How do you know that your private address has been translated into a public address?