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IP Addressing and Subnetting Interview Questions for CCNA

25 IP Addressing and Subnetting questions with answers for ccna roles — 9 easy, 12 medium, 4 hard.

About IP Addressing and Subnetting

Subnetting is the one skill every CCNA interviewer tests on paper. This topic covers IPv4 classes and private ranges, block-size subnetting, VLSM design, summarization, /31 links, ARP, and IPv6 addressing with EUI-64, SLAAC and Neighbor Discovery. Practise until you can find network, broadcast and host range for any prefix in under a minute.

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01

What are the IPv4 address classes and private ranges?

IP Addressing and SubnettingEasy
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Class A is 1 to 126 with a default /8, Class B is 128 to 191 with /16, Class C is 192 to 223 with /24, Class D 224 to 239 is multicast and Class E is reserved. RFC 1918 private ranges are 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16. 127.0.0.0/8 is loopback and 169.254.0.0/16 is APIPA link-local. Today addressing is classless (CIDR).
02

How many usable hosts are in a /26 subnet and what is the block size?

IP Addressing and SubnettingEasy
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A /26 leaves 6 host bits, so 2 to the power 6 is 64 addresses and 62 usable hosts after removing network and broadcast. The mask is 255.255.255.192 and the block size is 256 minus 192, which is 64. So subnets of 192.168.1.0/26 start at .0, .64, .128 and .192. For 192.168.1.70/26 the network is .64 and broadcast .127.
03

What is the structure of an IPv6 address and how is it shortened?

IP Addressing and SubnettingEasy
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IPv6 addresses are 128 bits written as eight groups of four hex digits separated by colons. Leading zeros in each group can be dropped, and one run of consecutive all-zero groups can be replaced with :: only once. For example 2001:0db8:0000:0000:0000:0000:0000:0001 becomes 2001:db8::1. A typical LAN uses a /64 prefix.
04

How does ARP work and what is a gratuitous ARP?

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When a host needs the MAC for an IP on its subnet, it broadcasts an ARP request and the owner replies with a unicast ARP reply; the result is cached. For remote destinations the host ARPs for its default gateway. A gratuitous ARP is an unsolicited announcement of a host's own IP-to-MAC mapping, used after failover, for example by HSRP, to update switch and host tables.
05

A host shows IP 169.254.12.5. What does this mean and what will you check?

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169.254.x.x is an APIPA address, assigned when the host fails to get a DHCP lease. I would check that the port is up and in the correct VLAN, that the DHCP server has free addresses in the scope, and that the router interface for that VLAN has ip helper-address pointing to the server if the server is remote. Then I run ipconfig /renew and check server logs.
06

A newly configured router interface rejects the IP 192.168.5.64/26. Why?

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With a /26 the block size is 64, so 192.168.5.64 is the network address of the subnet .64 to .127 and cannot be assigned to a host or interface. IOS reports a bad mask or overlap error. Use a usable address such as 192.168.5.65 instead. Similarly .127 would be the broadcast. Always calculate the subnet boundary before configuring addresses.
07

How do you configure IPv4 and IPv6 on a router interface?

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Enable IPv6 routing globally, then add both addresses to the interface. ipv6 unicast-routing interface GigabitEthernet0/0 ip address 192.168.1.1 255.255.255.0 ipv6 address 2001:db8:1:1::1/64 ipv6 address fe80::1 link-local no shutdown Verify with show ip interface brief and show ipv6 interface brief.
08

How do you verify IP settings and test from a Windows and Linux host?

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Windows and Linux both have a small core toolkit. ipconfig /all (IP, mask, gateway, DNS, DHCP server) ipconfig /release && ipconfig /renew arp -a ip addr ; ip route ; ip neigh resolvectl status nslookup example.com Add ping and tracert or traceroute to test the path.
09

How do you keep your subnetting skills sharp for interviews and work?

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Situation: in my first job, subnetting questions slowed me down during tickets. Task: get fast and accurate. Action: I practised ten random subnetting problems daily on a timer, memorised block sizes, and built my own spreadsheet for VLSM checks. I also reviewed real routing tables at work. Result: I solve most problems in under 30 seconds and now help juniors with IP planning during change reviews.
10

Find the network, broadcast and usable range for 172.16.45.200/21.

IP Addressing and SubnettingMedium
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A /21 mask is 255.255.248.0, so the interesting octet is the third with block size 8. 45 falls in the block starting at 40 (40 to 47). Network: 172.16.40.0 Broadcast: 172.16.47.255 Usable: 172.16.40.1 - 172.16.47.254 Hosts: 2^11 - 2 = 2046
11

What is VLSM and why is it used?

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Variable Length Subnet Masking means using different prefix lengths inside one address block, so each subnet is sized to its need. You allocate the largest subnets first, then smaller ones, to avoid overlaps. For example, from 10.1.1.0/24 you could give a 100-user LAN a /25, a 50-user LAN a /26, and point-to-point WAN links /30 or /31. It saves address space compared with fixed-length subnetting.
12

You have 192.168.10.0/24 and need subnets for 100, 50, 20 hosts and two point-to-point links. Design it with VLSM.

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Allocate largest first so blocks align. 100 hosts -> /25: 192.168.10.0/25 (.1 - .126) 50 hosts -> /26: 192.168.10.128/26 (.129 - .190) 20 hosts -> /27: 192.168.10.192/27 (.193 - .222) Link 1 -> /30: 192.168.10.224/30 Link 2 -> /30: 192.168.10.228/30 Free for growth: 192.168.10.232 - .255
13

What is route summarization and how do you calculate a summary?

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Summarization advertises one prefix that covers several contiguous networks, shrinking routing tables and hiding instability. Write the networks in binary and keep the common leading bits. For 10.1.4.0/24 to 10.1.7.0/24, the third octet values 4 to 7 share the first six bits (000001), so the summary is 10.1.4.0/22. Make sure the summary does not include networks that live elsewhere.
14

Why are /31 subnets used on point-to-point links?

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RFC 3021 allows /31 on point-to-point links because there is no need for network and broadcast addresses when only two devices exist. A /31 gives exactly two usable addresses, saving half the space of a /30. It is common on ISP and data center fabric links. Both ends must support it; on IOS you simply configure ip address 10.0.0.0 255.255.255.254.
15

Explain the main IPv6 address types.

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Global unicast (2000::/3) is publicly routable. Unique local (fc00::/7, in practice fd00::/8) is like private IPv4. Link-local (fe80::/10) is auto-created on every IPv6 interface and used for neighbour discovery and routing next hops. Multicast is ff00::/8, for example ff02::1 all nodes and ff02::2 all routers. Anycast is a unicast address shared by several devices. IPv6 has no broadcast.
16

What is SLAAC and how is it different from DHCPv6?

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With SLAAC a host learns the /64 prefix from a router advertisement and builds its own interface ID, without a server. Stateless DHCPv6 adds extra options such as DNS while the address still comes from SLAAC. Stateful DHCPv6 assigns the address itself and keeps bindings, similar to DHCPv4. RA flags M (managed) and O (other config) tell hosts which method to use.
17

Two hosts are in the same VLAN but one has mask /24 and the other /25. What problems may occur?

IP Addressing and SubnettingMedium
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Each host decides local or remote using its own mask. With 192.168.1.10/24 and 192.168.1.200/25, the first thinks .200 is local and ARPs directly, but the second sees .10 as remote and sends replies to its gateway. Traffic can become one-way or asymmetric and may break completely if the gateway does not route it back. Fix the mask so both match the design.
18

Your company is running out of IP addresses on a /24 user subnet. What options do you have?

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Options: expand to a /23 if the adjacent block is free and update the router and DHCP scope; create a new VLAN and subnet for some users, which also limits broadcast size; shorten DHCP lease times if many guest devices hold leases; and move printers, phones and IoT devices to their own subnets. Long term, plan an addressing scheme with growth headroom and consider dual-stack IPv6.
19

How do you quickly calculate how many subnets and hosts a mask gives?

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Use two formulas and memorise the mask octets. Subnets = 2^(borrowed bits) Hosts = 2^(host bits) - 2 Example: 10.0.0.0/8 to /20 -> 2^12 = 4096 subnets, 2^12 - 2 = 4094 hosts Mask octets: 128 192 224 240 248 252 254 255 Block size = 256 - mask octet
20

Describe a time you made a configuration mistake. How did you handle it?

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Situation: I applied a /25 mask instead of /24 on a core VLAN interface during a change window. Task: fix it fast and own the error. Action: monitoring alerted on half the users, I checked my change, corrected the mask within ten minutes and informed my lead immediately. Result: impact was short, and I added a peer-review step and a pre-written rollback to every change plan afterwards.
21

How do you mentor junior engineers who struggle with subnetting and troubleshooting?

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I start with the block-size method instead of binary tables, because it is fast and easy to remember. I give them daily five-minute subnetting drills, then real tickets where I let them lead while I watch and ask questions. For example, one junior engineer kept guessing during outages, so I taught him a fixed bottom-up checklist. Within two months he was closing Layer 2 and Layer 3 tickets on his own and teaching the next new joiner.
22

What is EUI-64 and how is the interface ID generated?

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EUI-64 builds a 64-bit interface ID from a 48-bit MAC. Split the MAC in half, insert FFFE in the middle and flip the seventh bit of the first byte. For MAC 0050.56AB.CDEF the ID becomes 0250:56FF:FEAB:CDEF. Routers can use ipv6 address 2001:db8:1::/64 eui-64. Hosts often prefer random privacy addresses instead, to avoid exposing the MAC.
23

How does IPv6 Neighbor Discovery replace ARP?

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NDP uses ICMPv6 messages. A host sends a Neighbor Solicitation to the target's solicited-node multicast address (ff02::1:ffXX:XXXX) and gets a Neighbor Advertisement with the MAC. Router Solicitation and Router Advertisement messages provide prefix and gateway. Duplicate Address Detection uses NS to check that an address is unique. Using multicast instead of broadcast reduces load on other hosts.
24

During a merger both companies use 10.0.0.0/16. How do you allow them to communicate?

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Overlapping address space means routing alone cannot work. Short term, use NAT on the interconnect so each side sees the other as a different translated range, for example twice NAT or policy NAT. Long term, renumber one network into a free range, which is cleaner but needs planning for DHCP, DNS, firewall rules and static IPs. I would also document the IP address management plan to prevent repeats.
25

How would you plan an IP addressing scheme for a company with 10 branches?

IP Addressing and SubnettingHard
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I would reserve a large block, for example 10.0.0.0/16, and give each branch a summarizable /20 such as 10.0.16.0/20, 10.0.32.0/20 and so on. Inside each, assign fixed ranges per role: users, voice, wireless, servers, management and WAN links. This lets each branch be summarized as one route, keeps firewall rules simple, and leaves room to grow. I keep it in an IPAM tool.

Frequently asked questions

How important is IP Addressing and Subnetting in CCNA interviews?

Subnetting is the one skill every CCNA interviewer tests on paper. This topic covers IPv4 classes and private ranges, block-size subnetting, VLSM design, summarization, /31 links, ARP, and IPv6 addressing with EUI-64, SLAAC and Neighbor Discovery. Practise until you can find network, broadcast and host range for any prefix in under a minute.

How should I use the IP Addressing and Subnetting sheet?

Go top to bottom — easy to hard. Answer before opening each answer, tick what you solved and save misses to Last-Minute Revision.

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