We have already discussed three private ranges used in classful and classless subnetting. The list does not stop there. This post identifies six types of special IP addresses that you will encounter. By the end of this post, you should have a clearer understanding of how these addresses are used in troubleshooting, how they appear on certification exams, and how they shape the way traffic moves across the Internet.
1. Loopback Address (127.0.0.1)
Every networked device has a loopback address. Most commonly, it is 127.0.0.1. The loopback address is typically used to test internal connections. You may wonder, “Why do I need to know if my device can connect to itself? Shouldn’t it connect to other resources and the Internet?”
Think about it this way. Before you walk out the door, you make sure you look presentable by taking a shower, brushing your teeth, ironing your clothes, and checking yourself in the mirror. Once everything looks good, you are ready to go.
Similarly, your computer uses the loopback address to check its internal network stack. Without it, the system would have difficulty testing and validating its internal network stack. This affects commands such as:
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ping
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tracert
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ipconfig
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curl
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nslookup
2. Multicast Addresses
Multicast addresses make up Class D, which ranges from 224.0.0.0 to 239.255.255.255. Multicasting sends data to multiple devices within a network. This one-to-many form of communication allows certain devices to join a group that will receive the traffic.
If you use Instagram, there is a feature called Close Friends. When you post a story, you can choose a specific group instead of all your followers. Only those selected see the post. That is similar to how multicasting works. Only devices that join the group receive the traffic.
Practical examples of multicasting include:
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Device casting such as AirPlay, Chromecast, and Roku
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OSPF, where routers exchange routing updates using multicast
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Digital signage at a restaurant displaying menus
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Live video streams
3. Broadcast Addresses
Broadcast addresses allow you to send data to all devices within a subnet. Each subnet is associated with a single broadcast address. Unlike multicast addressing, broadcast addresses are not limited to a specific class of IP addresses. If multicast addressing is like Instagram’s Close Friends feature, broadcast addresses are like posting a public story that everyone can see.
Broadcast traffic does not leave the subnet. For example, if a device sends a ping to the broadcast address, all other devices within the same subnet respond, including the gateway. When broadcast traffic reaches the gateway, it is typically dropped and not forwarded beyond the subnet.
Devices commonly use broadcast addresses to request services when the destination is unknown. Broadcast traffic may be sent to a specific subnet's broadcast address or to 255.255.255.255, known as the limited broadcast address, which is never forwarded beyond the local network. This includes:
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SSID broadcasting for wireless networks
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DHCP Discover messages
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Network share discovery
4. APIPA Address (169.254.x.x)
A device may receive an APIPA, or Automatic Private IP Addressing, address when it is unable to obtain an IP address from a DHCP server. APIPA addresses are automatically assigned and allow limited communication on the local network only. This is known as link-local communication and does not allow access to the Internet.
This screenshot shows a VMware network adapter configured as a host-only adapter with DHCP disabled. As a result, the virtual machine assigns itself an APIPA address of 169.254.209.154. The APIPA address range is 169.254.0.0/16. When DHCP is enabled, virtual machines using the VMnet1 adapter instead receive an IP address from the 192.168.146.0 subnet.
5. Default Route (0.0.0.0/0)
A routing table stores destination subnets used to forward packets. When a packet reaches a router, the router examines the destination IP address and checks for a matching route. If multiple routes match, the router selects the one with the longest matching prefix.
If no matching route exists, the router can:
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Drop the packet
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Use a configured default route to forward the packet
Both options come with different operational and security considerations. A default route provides a catch-all path when no specific route exists. Since the CIDR value is /0, the subnet mask is 0.0.0.0, which matches the entire IPv4 address space.
The drawback is that any traffic not explicitly defined in the routing table is forwarded to the next hop. Hosts commonly use a default gateway, which limits routing decisions to a single upstream device rather than the entire network.
6. Carrier Grade NAT
NAT translates private IP addresses into public IP addresses. Within a SOHO network, this conserves valuable IPv4 address space. You are not the only one who uses NAT. In many residential Internet setups, ISPs may also use a form of NAT where multiple customers share a public routable IP address, and traffic is differentiated using Port Address Translation (PAT). This is known as carrier-grade NAT (CGNAT).
When you access the Internet, your device uses its private IP address to reach its internal default gateway. The home router performs NAT and forwards the packet into the ISP network. This single-layer NAT approach offers simpler connectivity. CGNAT introduces an additional layer of NAT that may exist within the ISP network, where the ISP performs further network and port address translation. When the packet reaches the destination site, the source address appears as a shared public IP address rather than a unique address assigned to the customer. Traffic is still differentiated by port numbers.
Commonly, CGNAT uses the 100.64.0.0/10 address range. You can test whether your ISP uses CGNAT by comparing your router’s WAN IP address with the public IP shown by sites such as whatismyipaddress.com. If they do not match, this may indicate that CGNAT is in use. Network utilities such as tracert or traceroute may also indicate CGNAT usage, although some ISPs do not expose this information.
Some challenges users may encounter with CGNAT include:
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Difficulty using port forwarding
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ISP-enabled rate limiting due to shared IP addresses
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Difficulty hosting public-facing servers or services
What’s Next?
IPv4 remains the predominant addressing scheme in many modern networks. Despite its limitations, many workarounds have allowed systems to continue operating with minimal disruption to everyday users. However, IPv4 has still reached its address limit.
Think of it like packing a moving truck. You can rearrange the boxes to make better use of the space, but once the truck is full, rearranging no longer helps. At that point, you either need another truck or you unload and make another trip.
The next post will review NAT, CGNAT, and other IPv4 workarounds. After exploring the “rearrangement” option, we will move on to the “new truck” solution, IPv6.
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