From experience, I can tell you what most IT companies are going to ask you if you apply for a job.
First, if your going for a corporate IT job, the interviewer is going to quiz you first on your knowledge of the OSI networking model. An IT tech who doesn't know the OSI model (at least a solid understanding of the first 3 layers as well as the application layer), won't get very far in the interview process.
Secondly, you will be quizzed on subnetting. I can't tell you how many interviews I had where the prospective employer handed me a piece of paper and a pen and told me to subnet an IP address. He would give me a network ID and tell me to subnet that address into 20 subnets with 32 hosts per subnet (just as an example), and to list...