Richard Hay: On RFC 6598, IPV4 Address Space Explained

Written by Richard Hay

Are you up on your IPv4 address space knowledge. Our Richard Hay breaks it down. There is a block of 4 million IP addresses now allocated. Here’s why.

You’ve heard of the Internet Assigned Numbers Authority (IANA) reserved IPV4 prefix for shared address space — it’s been around since March 2012.

So let’s talk about the RFC 6598 net block of addresses for use as a private IP network.

It lets you take that net block of IP addresses and use them privately within intranets and internal lab networks the world over.  A lot of folks wonder what the range of this new net block is. The numeric range is:

If you don’t get the syntax above, no worries. It’s easy to break down.

First, note that the /10 is short for a subnet range.  That means the first 10 bits of the address is locked in as network bits. The other 22 are free for use by end nodes within that network — that of course brings the total up to 32 bits, which is the size of an IPv4 address.

Another way to say /10 is to represent it using dotted decimal as for the subnet mask. It is just shorter to write it as/10

So that mask basically slices off addresses starting with and marks almost 4.2 million addresses up to for use in this net block.

Now let’s talk more about the IANA IPv4 space registry.

That list shows the net block  — that’s where ARIN carves out the

Some simple nslookup queries on and yielded various Verizon named servers in the front end of the net block, for example. So it appears Verizon is using –

Now, there is an older RFC 1918 released way back in February 1996. It created three now commonly used private networks. In fact, if you’re reading this at home on a private network, you’re probably using one of them. Those are: — 65536 addresses total — 1 million addresses total — 16 million addresses total

So what’s the big deal? It might not seem relevant, but it is. Small businesses, for one thing, might find an extra network useful.

The other networks are so widely used that even small business could encounter a situation where is used by their ISP for their business connection and maybe the space is used internally. For larger companies, a new network is even handier. It’s hard to remember which 10 network the admin assigned to, say, the finance department, or sales and marketing.

But with the new scheme, it’s possible to assign and differentiate them. In the end, you avoid confusion and conflicts.

For larger companies the defined networks are often heavily in use and having another usable netblock actually is quite useful for internal intranets and allocation.

Anyway, not everyone peruses the IETF RFCs as a hobby or needs to know which netblocks you can use in a lab and not accidentally flood test packets at some actual website on accident.

So in case you missed the memo, here is your friendly neighborhood network engineer reminding you to stay safe and network, network, network.

For, I’m Richard Hay.

Image Credit: Wikimedia Commons



  • I forgot to mention another issue with the RFC 1918 addresses is that sometimes older network elements implement use of those subnets poorly and preclude you from using some of them (more than 1 sometimes) because of a crappy software coding.

    So in that kind of a case, the use of an alternative range can be useful so as not to conflict with that kind of a situation (though I would argue the better path is to lose the crappy element)