Generally, when you see names instead of IP Addresses, they are called, in general "domain names". Thus:
example.com is a domain name
sub.example.com is also a domain name
srv.sub.example.com is also a domain name
.com part is called a TLD (Top Level Domain). The
example part is called 2LD (Second Level Domain) and so on. The higher level domains are generally called "subdomains" of the lower level ones. Thus, referring to the examples above:
sub is a subdomain of
srv is a subdomain of
Now, in a URI, the parts after the slash after the domain name, is handled by the server who is using the domain name. For example, if we have the URI
http://sub.example.com/about, the browser contacts the server called
sub.example.com using the HTTP protocol, informing the server that the browser wants to view an object located in
/about (note the leading slash).
Is it possible to change the URI from
http://sub.example.com/about to, let's say
http://about.sub.example.com? Yes, but one needs a different IP address to ensure that what's returned by the latter is different from what's returned by
http://sub.example.com. The reason is that HTTP version 1.0 don't provide the full URI in its request, only HTTP version 1.1 actually provide the full URI. Thus, a browser not supporting HTTP version 1.1 will cause the server to be confused and likely receives the wrong page.