Private network

Key takeaways

Private network belongs to computer science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Private network to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Private network from memory before moving on to harder problems.

Reference excerpt

In Internet networking, a private network is a computer network that uses a private address space of IP addresses. These addresses are commonly used for local area networks (LANs) in residential, office, and enterprise environments. Both the IPv4 and the IPv6 specifications define private IP address ranges. 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, smartphone, or other Internet-connected device. 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. Private network addresses are not allocated to any specific organization. Anyone may use these addresses without approval from regional or local Internet registries. Private IP address spaces were originally defined to assist in delaying IPv4 address exhaustion. IP packets originating from or addressed to a private IP address cannot be routed through the public Internet. Private addresses are often seen as enhancing network security for the internal network since use of private addresses internally makes it difficult for an external host to initiate a connection to an internal system.

Private IPv4 addresses The Internet Engineering Task Force (IETF) has directed the Internet Assigned Numbers Authority (IANA) to reserve the following IPv4 address ranges for private networks. In practice, it is common to subdivide these ranges into smaller subnets.

Dedicated space for carrier-grade NAT deployment

In April 2012, IANA allocated the 100.64.0.0/10 block of IPv4 addresses specifically for use in carrier-grade NAT scenarios.

This address block should not be used on private networks or on the public Internet. The size of the address block 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.

Private IPv6 addresses

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 (ULAs). 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.

Link-local addresses

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.

IPv4 In IPv4, the utility of link-local addresses is in zero-configuration networking when Dynamic Host Configuration Protocol (DHCP) services are not available and manual configuration by a network administrator is not desirable. The block 169.254.0.0/16 was allocated for this purpose. If a host on an IEEE 802 (Ethernet) network cannot obtain a network address via DHCP, an address from 169.254.1.0 to 169.254.254.255 may be assigned pseudorandomly. The standard prescribes that address collisions must be handled gracefully.

IPv6 In IPv6, the block fe80::/10 is reserved for IP address autoconfiguration. The implementation of these link-local addresses is mandatory, as various functions of the IPv6 protocol depend on them.

Loopback interface A special case of private link-local addresses is the loopback interface. These addresses are private and link-local by definition since packets never leave the host device. IPv4 reserves the entire class A address block 127.0.0.0/8 for use as private loopback addresses. IPv6 reserves the single address ::1. An Internet Draft advocating reducing 127.0.0.0/8 to 127.0.0.0/16 was submitted in 2021; it expired in 2025.

Misrouting 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 black hole 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.

… excerpt ends here. Continue reading the full article.

Worked examples

Example 1 — a first encounter with Private network

Start with the simplest possible case. Write down what Private network claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In computer science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Private network before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Private network ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Private network

In research: Private network appears in computer science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Private network in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Private network is common in secondary-school and first-year university syllabi. It links to neighbouring topics IP addresses, Internet architecture, so understanding it makes those chapters shorter.
In everyday life: Look for Private network outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Private network in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Private network means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Private network out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Private network in simple terms?

In Internet networking, a private network is a computer network that uses a private address space of IP addresses. These addresses are commonly used for local area networks (LANs) in residential, office, and enterprise environments.

Why does Private network matter?

Because it connects several computer science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Private network?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Private network.