Private network

The Internet Engineering Task Force (IETF) has directed the Internet Assigned Numbers Authority (IANA) to reserve the following IPv4 address ranges for private networks:[1]^(p4)

In practice, it is common to subdivide these ranges into smaller subnets.

In April 2012, IANA allocated the block 100.64.0.0/10 (100.64.0.0 to 100.127.255.255, netmask 255.192.0.0) for use in carrier-grade NAT scenarios.[4]

This address block should not be used on private networks or on the public Internet. The size of the address block (2²², approximately 4 million addresses) was selected to be large enough to uniquely number all customer access devices for all of a single operator's points of presence in a large metropolitan area such as Tokyo.[4]

The concept of private networks has been extended in the next generation of the Internet Protocol, IPv6, and special address blocks are reserved.

The address block fc00::/7 is reserved by IANA for Unique Local Addresses (ULA).[2] They are unicast addresses, but contain a 40-bit random number in the routing prefix to prevent collisions when two private networks are interconnected. Despite being inherently local in usage, the IPv6 address scope of unique local addresses is global.

The first block defined is fd00::/8, designed for /48 routing blocks, in which users can create multiple subnets, as needed.

Examples:

A former standard proposed the use of site-local addresses in the fec0::/10 block, but because of scalability concerns and poor definition of what constitutes a site, its use has been deprecated since September 2004.[5]

Another type of private networking uses the link-local address range. The validity of link-local addresses is limited to a single link; e.g. to all computers connected to a switch, or to one wireless network. Hosts on different sides of a network bridge are also on the same link, whereas hosts on different sides of a network router are on different links.

Private addresses are commonly used in residential IPv4 networks. Most Internet service providers (ISPs) allocate only a single publicly routable IPv4 address to each residential customer, but many homes have more than one computer or other Internet connected device, such as smartphones. In this situation, a network address translator (NAT/PAT) gateway is usually used to provide Internet connectivity to multiple hosts.

Private addresses are also commonly used in corporate networks, which for security reasons, are not connected directly to the Internet. Often a proxy, SOCKS gateway, or similar devices are used to provide restricted Internet access to network-internal users.

In both cases, private addresses are often seen as enhancing network security for the internal network, since use of private addresses internally makes it difficult for an Internet (external) host to initiate a connection to an internal system.

It is common for packets originating in private address spaces to be misrouted onto the Internet. Private networks often do not properly configure DNS services for addresses used internally and attempt reverse DNS lookups for these addresses, causing extra traffic to the Internet root nameservers. The AS112 project attempted to mitigate this load by providing special blackhole anycast nameservers for private address ranges which only return negative result codes (not found) for these queries.

Organizational edge routers are usually configured to drop ingress IP traffic for these networks, which can occur either by misconfiguration, or from malicious traffic using a spoofed source address. Less commonly, ISP edge routers drop such egress traffic from customers, which reduces the impact to the Internet of such misconfigured or malicious hosts on the customer's network.

Since the private IPv4 address space is relatively small, many private IPv4 networks unavoidably use the same address ranges and hence the same addresses. This can create a problem when merging such networks, as multiple devices are likely to have the same address. In this case, networks or hosts must be renumbered, often a time-consuming task, or a network address translator must be placed between the networks to translate or masquerade the duplicate addresses.

For IPv6, RFC 4193 defines unique local addresses, providing an extremely large private address space from which each organization can randomly or pseudo-randomly allocate its own 40-bit prefix, each of which allows 65536 organizational subnets. With space for about one trillion (10¹²) prefixes, it is extremely unlikely that two network prefixes in use by different organizations are the same, provided each of them was allocated randomly, as specified in the standard. When two such private IPv6 networks are connected or merged, the risk of an address conflict is therefore virtually absent.

Despite official warnings, historically some organizations have used other parts of the reserved IP addresses for their internal networks.

• RFC 1918 – Address Allocation for Private Internets
• RFC 2036 – Observations on the use of Components of the Class A Address Space within the Internet
• RFC 7020 – The Internet Number Registry System
• RFC 2101 – IPv4 Address Behaviour Today
• RFC 2663 – IP Network Address Translator (NAT) Terminology and Considerations
• RFC 3022 – Traditional IP Network Address Translator (Traditional NAT)
• RFC 3330 – Special-Use IPv4 Addresses (superseded)
• RFC 3879 – Deprecating Site Local Addresses
• RFC 3927 – Dynamic Configuration of IPv4 Link-Local Addresses
• RFC 4193 – Unique Local IPv6 Unicast Addresses
• RFC 5735 – Special-Use IPv4 Addresses (superseded)
• RFC 6598 – Reserved IPv4 Prefix for Shared Address Space
• RFC 6890 – Special-Purpose IP Address Registries

• Heartbeat network
• Intranet
• Localhost
• Reserved IP addresses