The number changes each time they connect. Computers on company networks that have access to the Internet via a network also have static IPs , and can be changed by a technician or expert user. If an attacker doesn't know your IP, they have a harder time seizing control of your machine. They can use the client application to perform a search through a range of IP addresses. This can be difficult because there are four billion possible IP addresses.
Most corporations have firewalls in place.
The best defense against Trojan horses is to follow safe computing practices. Don't download or run programs from unknown sources. Symantec's Anti Virus Research Center has catalogued known Trojan horses and offers manual deletion solutions to them. Of course they need to be updated with virus signature files to catch the latest Trojan horse variants that are released periodically. These sig files can be obtain from the program vendor via the Internet.