These are common basic networking questions that interviewers ask to establish that you know the basics you’ll need for working with private networks (both in on-premise infrastructure and in the cloud):
What is a private network?
A private network is a network block (a range of IP addresses) that is not supposed to be publicly routable. These IP addresses are not magical in any way; it’s just an established convention that routers should not forward their traffic across private/public network boundaries. The effect is that these address blocks are safe to use for LANs and “private” cloud networks that are accessed locally, via VPNs, or via network peering in the cloud (which kind of breaks the ‘no routing’ rule but we’ll ignore that for now).
You might use a private network CIDR for your home network, or for workstations at your office, or for a part of your cloud infrastructure.
Which CIDR addresses denote private networks?
A private network is any network that uses an IP address space represented by one of the following CIDR blocks:
- 10/8 (e.g. 10.1.2.3)
- 172.16/12 (e.g. 172.18.1.2)
- 192.168/16 (e.g. 192.168.1.2)
You’ll notice they come in different sizes (a /8 is much larger than a 16) so they fit different purposes.
What is RFC1918?
RFC 1918 is an RFC (request for comments) which defines private networks — networks that should not be publicly routable.
RFCs are the mechanism that the Internet Engineering Task Force (IETF) uses for proposing and commenting on changes to the Internet and its protocols — a quick overview can be found here, on the official IETF site.