However, IP addresses are running out. Fast. A lot of ISPs and registration agencies are now not allocating whole Class Cs to companies at all, but only giving them the minimum number of addresses they actually need.
Certain web servers check this extra information, and can use it to decide between several sets of web pages, even if there is only one IP address being used. This is sometimes called software virtual hosting, or name-based virtual hosting, and is more efficient use of a diminishing resource, albeit at the expense of compatibility with some (mostly older) browsers. Increasingly, web-hosting companies and ISPs are only offering software virtual hosting, or charge a premium for hardware virtual hosting.
Host: full.domain.nameeg. when requesting the URL http://www.example.com/index.html, the browser should send:
GET /index.html HTTP/1.0 Host: www.example.com User-Agent: Mozilla/3.0 (compatible)and so on.
There is no disadvantage in a browser sending this information, since computers which use traditional hardware virtual hosting will simply ignore it. The advantage though is that the browser can then retrieve pages from computers which use software virtual hosting. As IP addresses continue to run out, the Net is likely to see more and more of such web sites. At least until IPv6 comes in, but that's a different story!