Reserved IPv4 addresses

IP Networks for Use in Private Networks (Local Networks, VPN):

Reserved IPv4 IP addresses that are not routed to the Internet, but are intended for building local networks and VPN networks. They are also called gray IP addresses (private IP addresses) because they are used inside local networks and are not intended for direct access from the global Internet. These addresses are defined in the RFC 1918 standard and are not routed in the public Internet.

• 10.0.0.0 — 10.255.255.255 (subnet mask for classless (CIDR) addressing: 255.0.0.0 or /8)
• 172.16.0.0 — 172.31.255.255 (subnet mask: 255.240.0.0 or /12)
• 192.168.0.0 — 192.168.255.255 (subnet mask: 255.255.0.0 or /16)

Usually, a Class C network with a mask of 255.255.255.0 (/24) is enough for building.

Danger: If you are setting up something more complex than home internet, do not use the networks below to avoid IP address conflicts from an accidentally enabled DHCP server on an unknown device:

• 192.168.88.0/24 is used by default in MikroTik;
• 192.168.0.0/24 is used by default in OpenWrt;
• 192.168.1.0/24 is used by default in D-Link.

Analysts claim that in a couple of years IPv4 will cease to exist, and a new, 6th version of the Internet protocol will take its place everywhere. However, I beg to differ with this opinion, which is backed by nothing other than “excessively multiplied Chinese people.” Even if ICANN and IANA wave the flag and drive central backbones into IPv6, many medium, and even more so small networks will continue to operate on the 4th version of the protocol. Tunneling and translation of IPv4 into IPv6 and back has not been canceled yet, look how many RFCs have been written on this matter, and all of them are entirely in the Standards Track, meaning they are quite mature, well-thought-out solutions. So, gentlemen, IPv4 will still last for our lifetime.

Therefore, I consider it quite appropriate to publish a relatively fresh IANA document (published as RFC 3330) on reserved IP addresses in the “reference” section. It would seem, what could be new here? “192.168/16, 10/8, 127.0.0.1… we already know them all by heart” — you might say. Sure, but how about another dozen? Do you know them by heart? And also all the RFCs that document which blocks are reserved for what purposes. The fact is that RFC3330 is the first document where all the information on this issue is gathered together. It’s even strange that IANA didn’t do this sooner.

Yes, and one more thing, regarding the “acute shortage” of IPv4 addresses: from the table provided below (on the next page), you will learn what address space was once reserved by IANA with implicit intentions or for projects that have long since faded into oblivion. Last year (specifically, RFC3330 was released in 2002), a strong-willed decision was made to gradually remove this “reservation” and distribute addresses from these blocks to all interested working people.

Moreover, if we turn to the IANA document “Internet Protocol v4 address space,” we learn that in addition to those listed in RFC3330, another 100 or so /8 blocks (what we used to call Class A networks, with 16,777,216 hosts per network) are hiding in IANA’s “bins.” Considering that, say, RIPE distributes approximately one /8 per year, and other registries about the same amount, the reserves will definitely last for another 20 years or so.

But we got a bit sidetracked… Let’s get back to our RFC3330. For simplicity and clarity, we decided to publish the document in an abridged form, discarding two prefaces (about the document’s status and about the leading role of IANA), and format the rest of the content as a table (the table is on the back of this sheet). It will make an excellent cheat sheet: you can tear out this sheet and hang it, for example, on the wall in the server room, it will turn out very convenient and nice 😉