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CCNA Subnetting Cheat Sheet

CCNA 200-301 exam notes · Updated 2026-10-06

Subnetting is the single topic most CCNA 200-301 candidates dread — and the one that pays off the most once it clicks. This CCNA subnetting cheat sheet distils the exam essentials: IPv4 subnetting, VLSM, route summarisation and IPv6 addressing, drawn from the NetExam Pro study guide. Read each section, drill the key points, then test yourself with our free CCNA mock test.

IPv4 Addressing and Subnetting

Every IPv4 address is 32 bits, written as four decimal octets, and it only means something paired with its subnet mask. Memorise the private ranges — 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16 — plus 127.0.0.0/8 (loopback) and 169.254.0.0/16 (APIPA, self-assigned when DHCP fails). The mask splits the address into network and host portions; the number of usable hosts in a subnet is always 2n − 2, where n is the number of host bits, because the all-zeros (network) and all-ones (broadcast) addresses can't be assigned.

For exam-speed subnetting, memorise the block sizes for the last octet: /24 = 256 (254 hosts), /25 = 128 (126), /26 = 64 (62), /27 = 32 (30), /28 = 16 (14), /29 = 8 (6), /30 = 4 (2). The trick is 256 minus the mask octet = block size: for 255.255.255.224, 256 − 224 = 32, so subnets increment in 32s (0, 32, 64, 96…). To find which subnet an address sits in, drop down to the nearest block-size multiple at or below the interesting octet — 192.168.1.77/27 sits in 192.168.1.64/27, with broadcast 192.168.1.95.

Practice the three question types until they're mechanical: given an address and mask, find the subnet, broadcast and usable range; given a host requirement, pick the smallest mask that fits (need 40 hosts → /26 gives 62, /27's 30 is too small); and given a subnet, list its address range. The exam rewards speed here, so drill with a timer rather than just reading about it.

Key exam points

  • Usable hosts = 2^n − 2 (n = host bits). Network and broadcast addresses are unusable.
  • Block size = 256 − mask octet (e.g. /27 → 32). Subnets step in block-size multiples.
  • Private: 10/8, 172.16/12, 192.168/16. Loopback 127/8. APIPA 169.254/16.
  • For 'smallest subnet for N hosts', pick the first mask whose usable count ≥ N.

VLSM and Route Summarisation

VLSM (Variable Length Subnet Masking) means using different mask lengths inside one address block so each subnet gets close to what it needs — no more, no less. The exam's golden rule: allocate the largest requirement first, then carve the next subnet out of what remains. Example: from 192.168.1.0/24, a 100-host LAN needs a /25 (192.168.1.0/25: .1–.126), a 50-host LAN needs a /26 (192.168.1.128/26: .129–.190), and a 20-host LAN needs a /27 (192.168.1.192/27: .193–.222). Starting with the small subnets first strands addresses in unusable gaps.

Route summarisation (aggregation) is the reverse skill: combining contiguous subnets into one advertisement. Find the bits the subnets share: 192.168.0.0/24, 192.168.1.0/24, 192.168.2.0/24 and 192.168.3.0/24 all share the first 22 bits, so they summarise to 192.168.0.0/22. The exam often asks for the best (longest) summary — including a subnet that doesn't exist in the range makes the summary wrong, so check the boundaries carefully.

Both skills rest on binary thinking, but there's a shortcut for summarisation: if you have 2n contiguous subnets of the same size aligned on the right boundary, the summary shortens the mask by n bits. Four aligned /24s → /22. Eight aligned /25s → /22. Always verify the first address (network) and the last broadcast fit inside your proposed summary.

Key exam points

  • VLSM rule: allocate largest subnets first to avoid stranded addresses.
  • Summarisation: combine contiguous subnets on the longest shared prefix.
  • 4 × /24 aligned → /22; each doubling of subnets shortens the mask by one more bit.
  • A summary is wrong if it includes addresses outside the original ranges.

IPv6 Address Structure

IPv6 addresses are 128 bits, written as eight 16-bit hextets in hexadecimal, separated by colons — for example 2001:0db8:0000:0000:0000:ff00:0042:8329. Two compression rules shorten this, and the exam tests both: drop leading zeros in each hextet (0db8 → db8, 0000 → 0), and replace the longest single run of all-zero hextets with :: — used only once per address. So the example becomes 2001:db8::ff00:42:8329. If two runs are equally long, compress the leftmost.

The prefix length works like an IPv4 mask but the standard LAN subnet is always a /64 — that leaves 64 bits for hosts, roughly 18 quintillion addresses per subnet. Know the common allocation sizes: /128 is a single host (loopbacks), /127 suits point-to-point links, /64 is the standard subnet, /56 gives a small site 256 subnets, /48 is the typical site allocation (65,536 subnets), and /32 is what ISPs receive from registries.

On Cisco routers, IPv6 routing is off by default — enable it with ipv6 unicast-routing, assign addresses with ipv6 address 2001:db8:1::1/64, and verify with show ipv6 interface brief and show ipv6 route. Remember that every IPv6 interface automatically gets a link-local address, which routing protocols use for next-hop communication.

Key exam points

  • 128-bit, 8 hextets. Compress leading zeros per hextet; use :: once for the longest zero run.
  • /64 is the standard subnet size. /48 = typical site, /56 = small site, /127 = point-to-point.
  • Enable routing with ipv6 unicast-routing; verify with show ipv6 interface brief.
  • Every interface auto-configures a link-local address (FE80::/10).

IPv6 Address Types and EUI-64

IPv6 has no broadcast — it uses scoped multicast instead. The types to know are: global unicast (2000::/3, the public internet range), unique local (fc00::/7, the private-address equivalent), link-local (fe80::/10, auto-configured on every interface, never routed), multicast (ff00::/8, replacing broadcast), plus ::1/128 (loopback) and ::/128 (unspecified). A host can — and usually does — hold several of these at once: link-local plus global, for example.

EUI-64 builds the 64-bit host portion from the 48-bit MAC address: split the MAC in half, insert FFFE in the middle, and flip bit 7 (the universal/local bit) of the first octet. Example: MAC 00:1a:2b:3c:4d:5e → 02:1a:2b:ff:fe:3c:4d:5e → interface ID 021a:2bff:fe3c:4d5e (00 became 02 because bit 7 flipped). Enable it with ipv6 address 2001:db8:1::/64 eui-64.

Address assignment comes in three flavours. SLAAC lets hosts self-configure from router advertisements (no DHCP server needed). Stateless DHCPv6 uses SLAAC for the address but a DHCPv6 server for DNS and other options. Stateful DHCPv6 hands out full addresses like DHCPv4. NDP (Neighbour Discovery Protocol) replaces ARP entirely, using ICMPv6 neighbour solicitation/advertisement messages — and it also handles router discovery and duplicate address detection.

Key exam points

  • Types: global 2000::/3, unique local fc00::/7, link-local fe80::/10, multicast ff00::/8, loopback ::1.
  • EUI-64: split MAC, insert FFFE, flip bit 7 of the first octet.
  • SLAAC = self-configured from router advertisements; stateful DHCPv6 = server-assigned addresses.
  • NDP replaces ARP in IPv6 (solicitation/advertisement, DAD, router discovery).

Related CCNA study guides

Frequently asked questions

What are the key CCNA exam points for IPv4 Addressing and Subnetting?

For the CCNA 200-301 exam, remember: Usable hosts = 2^n − 2 (n = host bits). Network and broadcast addresses are unusable. Block size = 256 − mask octet (e.g. /27 → 32). Subnets step in block-size multiples. Private: 10/8, 172.16/12, 192.168/16. Loopback 127/8. APIPA 169.254/16. For 'smallest subnet for N hosts', pick the first mask whose usable count ≥ N.

What are the key CCNA exam points for VLSM and Route Summarisation?

For the CCNA 200-301 exam, remember: VLSM rule: allocate largest subnets first to avoid stranded addresses. Summarisation: combine contiguous subnets on the longest shared prefix. 4 × /24 aligned → /22; each doubling of subnets shortens the mask by one more bit. A summary is wrong if it includes addresses outside the original ranges.

What are the key CCNA exam points for IPv6 Address Structure?

For the CCNA 200-301 exam, remember: 128-bit, 8 hextets. Compress leading zeros per hextet; use :: once for the longest zero run. /64 is the standard subnet size. /48 = typical site, /56 = small site, /127 = point-to-point. Enable routing with ipv6 unicast-routing; verify with show ipv6 interface brief. Every interface auto-configures a link-local address (FE80::/10).

What are the key CCNA exam points for IPv6 Address Types and EUI-64?

For the CCNA 200-301 exam, remember: Types: global 2000::/3, unique local fc00::/7, link-local fe80::/10, multicast ff00::/8, loopback ::1. EUI-64: split MAC, insert FFFE, flip bit 7 of the first octet. SLAAC = self-configured from router advertisements; stateful DHCPv6 = server-assigned addresses. NDP replaces ARP in IPv6 (solicitation/advertisement, DAD, router discovery).

Which key detail about IPv4 Addressing and Subnetting should you memorise for the CCNA 200-301 exam?

Block size = 256 − mask octet (e.g. /27 → 32). Subnets step in block-size multiples.