What is a C-block?

The definition of a C-block, and why it's important for SEO

A C-block is part of an IP-address. IP-addresses consist of 4 “blocks” of numbers, divided by dots. For example:

The first three blocks are referred to with A, B or C, like: AAA.BBB.CCC.001-255.

The C-block usually refers to the number of a webhost. So, webhost A might have a C-block of AAA.BBB.001.xxx, and webhost B AAA.BBB.002.xxx.

C-Blocks and SEO

If two resellers of the same host sell you two hosting accounts, there is a good chance they both have the same C-block.

Google assumes that sites hosted by two different hosts are probably separate, and therefore links between sites hosted on them are more likely to be from different people. There are problems with that assumption, but it’s one of the things they look at anyway (gotta look at *something*).

Let’s say you had an account with a shared IP address. So, for example, you had two sites that both used as an IP. Google would tend to assume that these two sites are related, since they are on the same IP. This can be an issue with free or cheap hosts, which may have thousands of websites hosted on the same shared IP. You would normally try to avoid this if you had multiple sites that were likely to link to each other.

Now let’s say that you got yourself 2 different (static) IP - your host would probably give you and, in this example. Well these are two different IP’s all right, but they are right next to each other, aren’t they? They do share the same C-block. Google would also likely consider these to be related.

But what if you hosted with another site across town? Perhaps they would be assigned a group of IP’s to hand out that look like 192.168.122.XXX. Well, that 122 now indicates a different ISP, and therefore two sites hosted at this level are more likely to be considered unrelated, as the C-blocks are different.

To make a long story short:

is a standard, fully qualified IP address. The blocks in this case are:


That’s not a Typo - Class D and E look totally different. The last 3 digits are actually called the Rest Field

So these have the same C-block:

And these have different C-blocks:

Note: A class C is not the only thing Google looks at to detect relationships - telling someone to “get a different class C-block” is not necessarily a fix for anything.

IP’s only become a real issue when you are looking at duplication issues, sites on related IP’s coming up for the same keywords, and for significant amounts of interlinking.

For most people, there are a lot better and more effective things to think about. You should only worry about IP’s if your rankings have given you a reason to worry about them, or if you are trying to spam the search engine (which I obviously don’t recommend!).

In short, you should be aware of some of the issues, but for someone to say that you NEED different Class “C”‘s is an exaggeration, IMO.

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