CCNA Subnetting: The Block-Size Method for v2.0 Troubleshooting Questions, With Worked Examples

Most CCNA subnetting mistakes aren't conceptual. Candidates know what a mask does and still lose time, or pick the wrong octet, when the question is wrapped in a troubleshooting scenario. The fix is one method you apply the same way every time, practiced until it takes seconds.

What CCNA objective 1.3 asks you to do with subnets

The v2.0 exam topics, which apply from February 3, 2027, list objective 1.3: "Troubleshoot IPv4 address configuration, assignment, and subnetting (public and private)." The v1.1 outline, in use through February 2, 2027, asks you to configure and verify IPv4 addressing and subnetting (1.6) and describe private IPv4 addressing (1.7). Either way, you calculate. v2.0 adds the step of finding what is wrong.

Subnet math also hides inside other objectives: reading a routing table (3.1), static routes (3.2), OSPF neighbors that must share a subnet and mask (3.3), DHCP pools and relay (1.7) and ACL wildcard masks (4.6). Cisco's CCNA FAQ, checked October 27, 2026, says that for people without IT experience the most challenging topics are often subnetting and the command line. Speed here pays off across the whole exam.

The CCNA mask table: values, block sizes and host counts

An IPv4 address has 32 bits in four octets. A mask octet can only take nine values: 0, 128, 192, 224, 240, 248, 252, 254 and 255. Learn the eight non-zero ones with their block sizes and you can subnet any prefix:

Usable hosts are 2 to the power of the host bits, minus 2. Cisco's guide Configure IP Addresses and Unique Subnets for New Users explains why: host IDs of all zeros (the network address) or all ones (the broadcast address) aren't assigned to devices. Two exceptions from the same guide matter for the CCNA:

• /31 has exactly two addresses and no network or broadcast address. RFC 3021 allows it on point-to-point links, which saves addresses on router-to-router links.
• /32 is a single address. Cisco's example is a loopback interface, which connects to nothing else.

The CCNA block-size method in four steps

1. Find the interesting octet: the first octet where the mask is neither 255 nor 0. For /27 it's the fourth octet (224). For /21 it's the third (248). With /8, /16 or /24 there isn't one: the network is the octets under the 255s, and the rest are zeros.
2. Compute the block size: 256 minus the mask value in that octet. 256 − 224 = 32.
3. Find the network address: in the interesting octet, take the largest multiple of the block size that is less than or equal to the address's value. Octets to the left stay as they are. Octets to the right become 0.
4. Find the broadcast and usable range: the next network minus one is the broadcast address (octets to the right become 255). Usable hosts run from the network address plus one to the broadcast address minus one.

That's the whole method. Explanations that walk through each step are what make it stick, which is why subnet math is worked step by step in the CCNA prep planned for launch.

Worked CCNA subnetting examples

All five examples are original, written by Passy for practice, and not taken from any Cisco exam.

Example 1: a host in a /27

Question: What are the network address, broadcast address and usable range for 10.14.200.77/27?

• The mask is 255.255.255.224, so the fourth octet is interesting and the block size is 256 − 224 = 32.
• Multiples of 32: 0, 32, 64, 96. The largest at or below 77 is 64.
• Network 10.14.200.64. Next network 10.14.200.96, so the broadcast is 10.14.200.95.
• Usable range 10.14.200.65 to 10.14.200.94: 30 hosts.

Example 2: a /21, where the third octet does the work

Question: Which subnet does 172.20.45.130/21 belong to, and how many hosts can it hold?

• The mask is 255.255.248.0. The third octet is interesting, block size 256 − 248 = 8.
• Multiples of 8 in the third octet: 40 is the largest at or below 45, and the next is 48.
• Network 172.20.40.0, broadcast 172.20.47.255, usable range 172.20.40.1 to 172.20.47.254.
• 32 − 21 = 11 host bits, so 2,048 − 2 = 2,046 usable hosts.

The fourth octet (130) plays no part in finding the network. That's the trap to watch with prefixes shorter than /24.

Example 3: a gateway outside the host's subnet

Question: A PC is set to 192.168.50.140 with mask 255.255.255.192 and default gateway 192.168.50.126. The router's interface for that VLAN is 192.168.50.126/26. Users report the PC can't reach other devices. Which setting is wrong?

• Block size for /26 is 64. The PC's address, 140, falls in the block that starts at 128: subnet 192.168.50.128/26, hosts .129 to .190.
• The gateway, .126, falls in the block that starts at 64: subnet 192.168.50.64/26, hosts .65 to .126.
• Keyed answer: the PC's address. A default gateway must be in the host's own subnet. The router interface defines the VLAN's subnet as 192.168.50.64/26, so the PC needs an address from .65 to .125.

Troubleshooting objectives reward this habit: every number looks plausible until you subnet both ends.

Example 4: a mask mismatch that ping won't show

Question: R1 G0/0 is 10.0.12.1/30 and R2 G0/0 is 10.0.12.2/29 on the same Ethernet link. Pings succeed both ways, but OSPF never forms a neighbor. Why?

• R1's /30 covers 10.0.12.0 to 10.0.12.3, which includes R2's address. R2's /29 covers 10.0.12.0 to 10.0.12.7, which includes R1's. So pings work.
• Cisco's OSPF neighbor troubleshooting guide lists the subnet and subnet mask among the hello parameters that must match.
• Keyed answer: the masks differ (/30 vs /29). On an Ethernet link left at OSPF's default broadcast network type, the routers won't become neighbors until both use the same mask.

The CCNA OSPF guide covers the other parameters neighbors must agree on.

Example 5: VLSM for four networks and two links

Question: Split 192.168.100.0/24 for Sales (100 hosts), Lab (50 hosts), Office (20 hosts) and two router-to-router links, with no overlap.

Allocate the largest requirement first, so smaller blocks never break the alignment of bigger ones:

Left free: 192.168.100.232/29 and 192.168.100.240/28. With /31 links instead, the two links would be 192.168.100.224/31 and 192.168.100.226/31.

Private, public and link-local addresses in CCNA troubleshooting

Objective 1.3 says "public and private." Cisco's addressing guide suggests reserved RFC 1918 addresses if you don't plan to connect to the internet. RFC 1918 reserves three blocks:

• 10.0.0.0/8: 10.0.0.0 to 10.255.255.255
• 172.16.0.0/12: 172.16.0.0 to 172.31.255.255
• 192.168.0.0/16: 192.168.0.0 to 192.168.255.255

The middle block catches people: 172.31.5.9 is private, but 172.32.5.9 is public. One more pattern to recognize when you troubleshoot client connectivity (objectives 1.6 and 1.7): a client showing a 169.254.x.x address has assigned itself a link-local address, a classic sign that it didn't get a DHCP lease.

Subnetting mistakes that cost CCNA points

• Using the wrong octet. With /17 to /23, the block size applies to the third octet, as in Example 2.
• Counting addresses instead of hosts. A /27 has 32 addresses and 30 usable hosts. Questions that say "hosts" mean usable ones.
• Off-by-one broadcasts. The broadcast is the next network minus one, not the next network.
• Assigning the network or broadcast address to a device. Fine only on a /31 point-to-point link.
• Planning VLSM smallest first. Small blocks placed first can leave a big requirement with no aligned space.
• Confusing masks with wildcards. ACLs use wildcard masks, the inverse: /27 becomes 0.0.0.31. The CCNA ACL guide covers them.

When the same slip shows up twice, it's a habit, not bad luck. Tracking it helps: missed subnetting questions wait in Fix It, planned for launch, until you get them right.

Building CCNA subnetting speed

A practical routine from Passy, not a Cisco recommendation:

1. Days 1 and 2: write the mask table from memory until it takes under a minute.
2. Days 3 to 5: 20 random addresses a day with random prefixes from /17 to /30. Type each answer instead of choosing it, so "close" doesn't count.
3. Days 6 and 7: troubleshooting-style items like Examples 3 and 4, where you subnet two devices and compare.
4. After that: fold subnetting into routing-table and ACL practice. The CCNA routing table guide uses the same math to pick a next hop.

The 20 free CCNA practice questions include a /27 gateway item, a longest-match routing item and an ACL wildcard item, each with a worked explanation. If subnetting is your weakest skill, name Network Infrastructure and Connectivity as the focus area in the free CCNA study planner. For a larger set, the 200 Network Infrastructure and Connectivity questions planned for launch span the whole domain, subnetting included. Checkout isn't open yet.