When you hear about web clustering, a few different technologies come to mind. The option you cite is what is typically called DNS Round-robin load balancing. With this, both IP addresses are listed as DNS
A records for the domain. Some clients will resolve one IP address and some will resolve the other. This is a very simplistic way of load balancing, and can usually only work on websites that don't maintain any state about the user's session.
The preferred method of load balancing is to have two or more webservers behind one large load balancer. In this case, the website IP address resolves to the IP of the load balancer, and then the load balancer proxies the user's requests to the back-end webservers, distributing the load between all of the active nodes as necessary.
For example:- There is a website
www.abc.com, which is resolved at
10.10.10.21:80,and on 10.10.10.22:80
Just for your information, DNS resolution has nothing to do with TCP (or UDP for that matter) port numbers. DNS will only resolve to an IP address, and then the application (web browser in this case) needs to know what port to connect to.