Is it really necessary to learn about the OSI layers? yes I'm afraid it is. I know it's a boring theory type, but they really useful for troubleshooting network and there can be a lot of questions in the CCNA exam related to OSI Layers.

OSI layers is a conceptual thing that urge all vendors for network devices to follow their concept so all devices from different vendors can communicate. Long time ago before this concept exist, the devices from one vendor can't communicate with other vendors'.

» Read more: OSI Layers in CCNA Exam

You started your network with small amount of computers and networking devices, sooner or later you want to add this and that, next thing you know your network got slower.

If you have this condition in your network, you might want to consider reviewing back you what devices you have in your network. You might need to segment your network.

You can use switches and bridges to segment your network, if you use them you separate your network collision domain. Remember what collision domain is when I posted about switches.

» Read more: Network Segmentation

Tenable Nessus from http://www.nessus.org is the tool of my choice if I want to scan my network for vulnerabilities.

Say like this, I want to know if my network is safe from dangerous people that want to mess up my network, I need to know are there any "holes" that can be an advantage to those people. So I run my tenable nessus from my PC, scan all computers in my network, and the nessus will provide me details of all vulnerabilities.
They'll also give you the details of each vulnerabilities, which ports are currently open, what's the meaning of it, is it dangerous, the links to the description, etc.

» Read more: Scan for Network Vulnerabilities