Cisco Certified Network Associate (CCNA) 200-301 · Free study guide

Objective 1.6 — Configure IPv4 addressing and subnetting

Subnetting is a boundary problem. A prefix length says how many leading address bits identify the network; the remaining bits vary within that subnet. Calculate the boundary before judging whether an address, gateway, or route belongs.

Prefixes and block size

IPv4 contains 32 bits. A /24 leaves eight host bits, a /26 leaves six, and a /30 leaves two. In conventional subnet questions, total addresses equal 2^(host bits). The subnet mask is the same boundary in dotted decimal. For example, /26 is 255.255.255.192; the last-octet block size is 64, producing boundaries at 0, 64, 128, and 192.

Do not subtract two before finding the network. First calculate the entire block, then identify network and broadcast boundaries and the conventional usable host range between them. Specialized modern uses can treat address semantics differently, but a scenario will usually make that clear.

Network, broadcast, and hosts

Apply the mask with the address to find the network. Set all host bits to one for the directed broadcast. For 192.0.2.77/26, the address lies in 192.0.2.64–127: network .64, broadcast .127, and conventional hosts .65 through .126. A gateway must be a usable address in the host's subnet.

Two addresses that look numerically close can be separated by a prefix boundary. Conversely, addresses in different final-octet ranges can share a larger prefix.

Variable-size planning

When allocating several subnets, reserve the largest requirement first, choose the smallest prefix that supplies enough addresses, align it on a valid boundary, and continue without overlap. Record network, prefix, usable range, broadcast, and purpose. Leave deliberate growth space rather than accidental gaps no one can explain.

For 50 conventional hosts, /26 supplies 64 total and 62 usable addresses. A /27 supplies only 30 usable and is too small. The requirement decides the prefix; memorized department sizes do not.

Interface configuration and verification

An IOS-style routed interface needs the selected address and mask and must be administratively enabled. Verification includes operational state, the assigned prefix, connected and local routing entries, and reachability to an appropriate neighbor. A correct address on a down interface does not create usable connected reachability.

Check for duplicate use, correct VLAN or physical attachment, and a matching peer prefix. A successful local ping does not prove remote routing.

IOS command map

On a lab interface, enter interface GigabitEthernet0/1, apply a planned address with ip address 192.0.2.65 255.255.255.192, and use no shutdown. The address and mask are one decision: substituting a mask from a different subnet changes the connected prefix.

Use show ip interface brief to check address and line state, show ip interface GigabitEthernet0/1 to inspect the interface in detail, and show ip route connected to confirm the expected connected route. Test one known on-link neighbor before testing a remote prefix. Interface names and available verification fields vary by IOS platform and release.

Worked scenario

Three LANs need 100, 45, and 12 conventional host addresses from a /24. Allocate the 100-host LAN a /25, the 45-host LAN a /26, and the 12-host LAN a /28, each on its proper boundary. Allocating the small LAN first without a plan can fragment the space and make the larger boundary unavailable.

Common traps

• Confusing total and conventional usable addresses.
• Starting a subnet on a non-boundary address.
• Comparing only the last octet instead of masked bits.
• Using a network or broadcast address on a host.
• Forgetting interface state and duplicate-address checks.

Readiness checklist

• I can convert common prefixes and masks.
• I can derive network, broadcast, and host range.
• I can allocate variable-size subnets without overlap.
• I can configure and independently verify an IPv4 interface.