IP addressing, explained for CompTIA Network+

What does Network+ test about IP addressing?

Network+ tests whether you can look at an address and know what it is, where it came from, and what to do next. On the N10-009 exam, IPv4 network addressing is objective 1.7, inside Domain 1.0 Networking Concepts, which is worth 23 percent of your score.

That means addressing questions appear often, and they reward understanding over rote memorization. The exam is scenario based, so it tends to hand you a symptom and ask for the address concept that explains it. You might be asked to spot a private range, recognize an address that means a device never heard back from a server, or choose the right gateway for a subnet. None of that requires binary gymnastics. It requires knowing a handful of ranges and roles, which is exactly what we cover below. We keep the subnetting math in a separate subnetting guide so this page stays focused on what each address is and what it signals.

What are an IPv4 address and a subnet mask?

An IPv4 address is a 32-bit number written as four decimal octets, such as 192.168.1.10. The subnet mask, written like 255.255.255.0, marks which part of that address is the network and which part is the host.

Each octet is one of the four numbers separated by dots, and each can range from 0 to 255. That range exists because every octet holds eight bits, and eight bits across four octets gives you the 32 bits in the whole address. The mask acts like a ruler laid over the address. Where the mask shows 255, that position belongs to the network. Where it shows 0, that position identifies the individual host. So 192.168.1.10 with a 255.255.255.0 mask sits on the 192.168.1 network as host number 10. You will also see masks written in shorthand, like /24, which simply counts how many bits belong to the network. Devices on the same network can talk to each other directly. To reach anything outside it, traffic needs a gateway, which we come to shortly. If you want to see how addresses ride on top of the lower layers, our OSI model walkthrough connects the pieces.

Public or private: which address goes where?

Public addresses are unique across the entire internet and routable everywhere. Private addresses are reserved for use inside your own network and are never routed on the public internet. Three private ranges, defined in RFC 1918, cover almost every home and office.

Because private addresses cannot travel the internet on their own, a router uses Network Address Translation, or NAT, to swap them for one shared public address on the way out. If you have ever seen a home router hand out 192.168.1.x, that is RFC 1918 at work. The full definition lives in RFC 1918, and committing these three ranges to memory is one of the highest-value things you can do for the exam.

• 10.0.0.0/8: 10.0.0.0 to 10.255.255.255, the largest private block, common in bigger networks.
• 172.16.0.0/12: 172.16.0.0 to 172.31.255.255, a mid-size block that many people forget.
• 192.168.0.0/16: 192.168.0.0 to 192.168.255.255, the range most home routers use.

Static or dynamic: how does a device get its address?

A device gets its address one of two ways. Static means a person types it in by hand and it stays put. Dynamic means a DHCP server hands it out automatically, along with the subnet mask, the default gateway, and the DNS server.

Most everyday devices use dynamic assignment through DHCP, because it scales without someone tracking every address. A laptop joins the network, asks for an address, and the DHCP server leases it one for a set time, renewing the lease quietly in the background. Servers, printers, and network gear often use static addresses instead, so their address is predictable and does not drift between reboots. In many offices you can even reserve a specific address for a device inside DHCP, which blends the convenience of dynamic with the predictability of static. Two roles ride along with the address and matter for the exam. The default gateway is the exit door: it is the router address a device sends traffic to whenever the destination sits on another network. The DNS server is the translator: it turns a name like example.com into the numeric address your device actually connects to. When either one is wrong or missing, a device can hold a perfectly valid address and still reach nothing useful.

What do 169.254 and 127.0.0.1 addresses mean?

These two addresses are messages. A 169.254.x.x address means the device asked for DHCP and got no answer, so it assigned itself a link-local address. And 127.0.0.1 is loopback, the address a device uses to talk to itself.

When a client cannot reach a DHCP server, it falls back to Automatic Private IP Addressing, or APIPA, and picks something in 169.254.0.0/16. That address lets it talk to other stranded devices on the same segment, but it cannot reach the gateway or the internet. So if you run ipconfig or a similar tool and see 169.254 anything, do not chase DNS or the browser. Look at the DHCP path: the cable, the switch port, and the DHCP server itself. The loopback address, 127.0.0.1, is a different signal. Anything in 127.0.0.0/8 points straight back at the local machine, and pinging 127.0.0.1 is a quick way to confirm the device's own TCP/IP stack is working before you blame the wider network. On the exam, both are recognition questions: see the range, name the meaning.

How is IPv6 different, and is NAT still needed?

IPv6 is the newer, much larger addressing system. An IPv6 address is 128 bits instead of 32, written in hexadecimal with colons, and it provides so many addresses that NAT is no longer needed to conserve them.

Where IPv4 has roughly four billion possible addresses, IPv6 has a supply large enough that every device can hold a globally unique address without translation. Adoption has grown steadily as the internet ran short of fresh IPv4 space, which is the real reason the exam expects you to recognize it. Addresses look like 2001:0db8:85a3::8a2e:0370:7334, where the double colon is shorthand for a run of zeros. IPv6 has its own link-local range too: anything starting with fe80 is a link-local address a device configures automatically to communicate on its local segment, similar in spirit to what APIPA does for IPv4. You do not need to master IPv6 subnetting for a plain-English start. For Network+, know that it is 128-bit, hexadecimal, needs no NAT, and uses fe80 for link-local. That foundation is enough to answer most concept questions and to keep building from there.

Putting IP addressing together for exam day

IP addressing rewards a small amount of memorization and a lot of pattern recognition. Know that an IPv4 address is 32 bits split by a subnet mask; that the RFC 1918 ranges 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16 are private; that DHCP hands out addresses while static assignment is set by hand; that 169.254 means no DHCP answer and 127.0.0.1 is loopback; and that IPv6 is the 128-bit future that skips NAT. Those facts cover the core of objective 1.7. Do not try to cram all of it at once, since a few short review sessions beat one long night. The fastest way to lock the ranges in is to see them repeatedly in questions, so run a set in Study mode and let them become automatic before test day.