Portable Block - the simple definition is an Internet Protocol Address (IP address) or range of IP addresses (block) that is reserved or assigned to a specific domain, user, or group.
Sorry - but for the rest of this explanation we'll need to get a little geeky to make the point!
All web sites have an Internet Protocol address (IP address). Actually, every computer in the world that is connected to the Internet has an IP address, including the computer you are using to read this right now! An IP address is composed of 4 groups of three numbers separated by dots. If the first or first and second number in the group is a 0, then it is not included in the address to save space. (Ex. 66.161.162.178 is really 066.161.162.178). These addresses must be unique for you to get where you are going, and for information to get back to your computer, and must remain assigned to you during the entire period you are connected to the Internet. If you disconnect from the Internet your IP address can be assigned to someone else, and when you re-connect to the Internet you may get a different IP address. This is called 'dynamic' IP addressing. But if you are a web site or have some kind of 'presence' on the Internet, your IP address must be the same all the time.
Here's and example, these two links are the same web site - click on the first one, then the back button on your browser to return here, then click on the other one and your back button on your browser to return here and continue:
So when you are typing www.anaturalplace.com you are really typing 66.161.162.178 . It's just easier to remember anaturalplace.com. So, to make using the Internet easier, when you type anaturalplace.com, the request goes across the internet to a computer called a Domain Name Server (or DNS) which translates the anaturalplace.com to the IP address and sends the request on for you. The IP address 66.161.162.178 is reserved for anaturalplace.com, or is 'ported' to that domain name.
Why am I bothering you with all of that? Well - you have to understand that for the rest to make sense!
A router or switch that provides DHCP addressing stops your IP address at the device, and translates it to the IP address range of your secured internal network. For the most part, that IP address range will be limited to 192.168.###.###. The addresses in the 192.168 range are 'non-portable' and are reserved for manufactures to have a range of IP addresses for DHCP addressing! So - if I would manage to fine your IP address and hack your switch or router, I would then be able to attack your machines more readily because I know they are in a relatively 'narrow' range of possible IP addresses.
BUT - most routers and switches can be configured to change this default range of IP addresses. If you change that default range to a range of IP addresses that are 'portable' or already 'ported', then when the intruder tries to hack machines in the 192.168 block - your machines just aren't there! If the intruder realizes that your DHCP address block is changed to a different block, well, every time he tries to attack an IP address in your range, his attack will be re-directed to the machines that are ported to that block in the 'real world', and he won't get to your secured network.
It's kind of security by obfuscation. This really won't stop a good hacker, but it will drive the wannabes away - and those make up the majority of the malicious hackers in the world. Considering that our goal here is to protect a home network, the odds of a hard core hacker attacking you are pretty remote.
The downside to the discussion is that it will make the configuration of your switch or router a little harder because you won't be able to follow the instructions 'to the letter', you'll have to substitute your 'ported IP range' in the instructions every time you see 192.168.###.###. The upside is that it's more free security, and you should take advantage of the easy stuff when you can!