A private IP address is an IPv4 address reserved for use inside private networks rather than as a globally unique Internet address. To identify one, check whether the full address falls within 10.0.0.0/8, 172.16.0.0/12 or 192.168.0.0/16. A duplicate is an IP conflict only when two devices use the same address on the same network link; matching numbers on separate networks are normal.
What makes an IP address private?
IPv4 private addresses are reusable within separate private networks. They identify devices within the network using them, but they are not globally unique Internet addresses. A router or other gateway may mediate a device’s access to external services. The three ranges reserved by RFC 1918 are:
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| Private IPv4 block | Full address range | Example |
|---|---|---|
10.0.0.0/8 |
10.0.0.0–10.255.255.255 |
10.12.3.4 |
172.16.0.0/12 |
172.16.0.0–172.31.255.255 |
172.20.4.8 |
192.168.0.0/16 |
192.168.0.0–192.168.255.255 |
192.168.1.25 |
The middle block is limited to addresses beginning with 172.16 through 172.31. For example, 172.20.4.8 is private, but 172.40.4.8 is not. The ranges are address categories, not a guarantee that a particular device is reachable or that a duplicate exists.
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How to tell whether your address is private
Find the full IPv4 address assigned to the network interface in question, then compare it with the ranges above. Do not rely only on the first two digits of an address, especially for the 172 block. If the address does not fit any of the three ranges, it is not an RFC 1918 private IPv4 address.
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169.254.x.x is link-local, not RFC 1918 private
An IPv4 address beginning with 169.254 belongs to the 169.254.0.0/16 link-local range described by RFC 3927. It is intended for communication on the same physical or logical link, not off-link communication. A device may use it when ordinary address configuration is unavailable; it is not an additional private IPv4 block or a publicly routable address. The RFC advises administrators configuring local addresses manually, through DHCP or another method to use an RFC 1918 prefix rather than 169.254/16.
IPv6 has a different local-address range
IPv6 unique-local unicast addresses come from FC00::/7. Under RFC 4193, they are intended for local communication and are not expected to be routed on the global Internet, although they may be routed within a site or between a limited number of sites. This IPv6 category is not interchangeable with the IPv4 ranges above.
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When is a duplicate an IP conflict?
An IP conflict means two hosts are using the same address on the same link. As RFC 5227 explains, devices on separate links can reuse a private address without conflict. A link is the local scope where hosts can exchange link-layer packets and receive broadcasts; switches and bridges can connect devices on a link, while an IP router separates links for this purpose.
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How to investigate and resolve a conflict
- Record the affected interface and address. Note the complete IPv4 address and identify the network adapter or connection reporting the issue.
- Check how the address is assigned. Determine whether the device is configured with a manual/static address or receives one through DHCP. The setting names and screens vary by operating system.
- Compare local assignments. Review the router or DHCP server’s leases and reservations alongside the network’s intended static-address assignments. Look for the same address assigned twice, or an address included in a dynamic pool while also configured manually.
- Correct the overlap. Following the network’s address plan, assign a unique address on that link or adjust the relevant DHCP pool, reservation or manual setting. On a managed network, ask the administrator before changing gateway or infrastructure settings.
- Reconnect or renew configuration as appropriate. Use the procedure for the device and operating system, then check whether the warning and connectivity problem have cleared. There is no single command or router menu that applies to every network.
If the device has a 169.254 address
Investigate why ordinary network configuration is unavailable, such as whether the device can obtain its intended configuration. Treat the address as link-local rather than as an RFC 1918 address. RFC 3927 requires hosts that configure IPv4 link-local addresses to be prepared to detect conflicts and choose another address; that automatic behavior applies to link-local configuration, not as a universal fix for every private-address conflict.
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What the address ranges do—and do not—tell you
- A private IPv4 address tells you the address is in a range reserved for reuse on private networks; it does not by itself identify a globally unique device.
- A link-local IPv4 address indicates a same-link scope and may point to unavailable ordinary configuration; it does not establish an RFC 1918 assignment.
- An IPv6 unique-local address is intended for local communication and can be routed within limited private scopes; it is not one of the IPv4 private ranges.
- The address category alone does not diagnose a conflict. The relevant check is whether another device on the same link is using that address.
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