Why is it that when I am connected to the internet my localhost does not connect?

Answer 1

The localhost is the loopback network interface. Every machine uses the exact same address, 127.0.0.1, but that address is completely private to your machine. Even your local area network cannot access it, never mind the internet. The localhost essentially allows you to refer to your own machine, permitting access to its own servers (such as http://127.0.0.1 or ftp://127.0.0.1) just as if the request had actually come from another machine on the network. Even completely standalone machines have a localhost.

Your machine may have one or more additional IP addresses, depending on the number of network interfaces available. Most home computer systems use a router to connect to the Internet and every machine connected to it forms a local area network (LAN). Typically the router's LAN IP address will be in one of three ranges where the first octet is: 1-126 (class A), 128-191 (class B) or 192-223 (class C). Most home networks use class C addresses where the first three octets identify the subnet (such as 192.168.0) and the last octet identifies the machine (1 to 254). The router is typically assigned the first IP address, such as 192.168.0.1, and this is the gateway to the Internet. LAN addresses are not accessible from outwith the LAN -- they are private to the LAN (just as localhost is private to the machine).

The router also has an outward facing IP address that is assigned to it by your ISP. This is the only address that is visible to the Internet. The router ensures that traffic from the LAN to the Internet is made to appear as though it originated from the router via network address translation (NAT). Basically, whenever you instigate an Internet request, the router receives the request and stores it in a table along with your LAN address. The router then modifies the request (changing the port or adding additional information to the request) in order make it unique to you, swaps your LAN address for your ISP address and completes the call. When a response is received, the router looks up the table for a matching entry, swaps the ISP address for your local address and forwards the call to your machine. This gives the impression your machine is directly connected to the Internet, but it is not. The Internet is connected to your router and nothing else.