CCNA 200-301 v1.1 reference — Network Fundamentals

IPv4 subnetting

Convert prefix lengths and masks, find network, broadcast and usable ranges, and handle /31 and /32 boundary cases correctly.

Objective 1.6v1.1 exam target

Quick Reference Handbook

The subnetting method

Block size

256 − mask octet

The interesting octet's step size, e.g. mask .192 → block of 64.

Usable hosts

2^hostbits − 2

Subtract network and broadcast — except /31 links and /32 hosts.

Network address

Address AND mask

First address of the block; not assignable on normal subnets.

Broadcast address

Network + block − 1

Last address of the block; not assignable.

Subnetting needs three numbers: the block size in the interesting octet, the network address (nearest multiple of the block size at or below the host octet), and the broadcast (network + block − 1). Everything else — first host, last host, host count — falls out of those three.

Prefix lengths, masks, wildcards and block sizes

Binary arithmetic, not globally allocatable host counts — reserved ranges and addressing policy still apply. /0 is normally the default route, /31 is point-to-point only (RFC 3021), /32 is a single host route.
Prefix	Dotted mask	Wildcard	Addresses per block	Theoretical host slots*
/0	0.0.0.0	255.255.255.255	4294967296	4294967294
/1	128.0.0.0	127.255.255.255	2147483648	2147483646
/2	192.0.0.0	63.255.255.255	1073741824	1073741822
/3	224.0.0.0	31.255.255.255	536870912	536870910
/4	240.0.0.0	15.255.255.255	268435456	268435454
/5	248.0.0.0	7.255.255.255	134217728	134217726
/6	252.0.0.0	3.255.255.255	67108864	67108862
/7	254.0.0.0	1.255.255.255	33554432	33554430
/8	255.0.0.0	0.255.255.255	16777216	16777214
/9	255.128.0.0	0.127.255.255	8388608	8388606
/10	255.192.0.0	0.63.255.255	4194304	4194302
/11	255.224.0.0	0.31.255.255	2097152	2097150
/12	255.240.0.0	0.15.255.255	1048576	1048574
/13	255.248.0.0	0.7.255.255	524288	524286
/14	255.252.0.0	0.3.255.255	262144	262142
/15	255.254.0.0	0.1.255.255	131072	131070
/16	255.255.0.0	0.0.255.255	65536	65534
/17	255.255.128.0	0.0.127.255	32768	32766
/18	255.255.192.0	0.0.63.255	16384	16382
/19	255.255.224.0	0.0.31.255	8192	8190
/20	255.255.240.0	0.0.15.255	4096	4094
/21	255.255.248.0	0.0.7.255	2048	2046
/22	255.255.252.0	0.0.3.255	1024	1022
/23	255.255.254.0	0.0.1.255	512	510
/24	255.255.255.0	0.0.0.255	256	254
/25	255.255.255.128	0.0.0.127	128	126
/26	255.255.255.192	0.0.0.63	64	62
/27	255.255.255.224	0.0.0.31	32	30
/28	255.255.255.240	0.0.0.15	16	14
/29	255.255.255.248	0.0.0.7	8	6
/30	255.255.255.252	0.0.0.3	4	2
/31	255.255.255.254	0.0.0.1	2	2 (point-to-point only)
/32	255.255.255.255	0.0.0.0	1	1 (host route)

Binary arithmetic, not globally allocatable host counts — reserved ranges and addressing policy still apply. /0 is normally the default route, /31 is point-to-point only (RFC 3021), /32 is a single host route.

Subnet 192.168.1.77/26

Address: 192.168.1.77
Prefix: /26

Mask: /26 → 255.255.255.192. Interesting octet is the 4th.
Block size: 256 − 192 = 64 → subnet boundaries at .0, .64, .128, .192.
77 falls in the .64–.127 block → network 192.168.1.64, broadcast 192.168.1.127.
Usable range: 192.168.1.65 – 192.168.1.126 → 62 usable hosts.

192.168.1.77/26 lives in 192.168.1.64/26, usable .65–.126, broadcast .127.

/31, /32 and wildcards

/31 is a special case defined by RFC 3021 for point-to-point links: with only two addresses, both are usable endpoints and there is no network/broadcast reservation — it saves space on router-to-router links. /32 identifies exactly one host; it appears as host routes and loopbacks. Applying 'minus 2' blindly to these prefixes gives −1 or wrong answers.

Wildcard masks are the bitwise inverse of a subnet mask: subtract each mask octet from 255. A /26 network uses mask 255.255.255.192 and wildcard 0.0.0.63; /24 uses wildcard 0.0.0.255; a single host is wildcard 0.0.0.0. Zero wildcard bits must match exactly; one bits are ignored — the same logic ACLs and OSPF network statements use.

Verification and mistakes

Verify subnetting on a device

Confirm interface addresses land on the intended subnet — Privileged EXEC

Cisco IOS / IOS XE

Verify

show ip interface brief
show ip interface gi0/1
ping <neighbor-ip>

Configured mask matches the intended prefix.
Neighbors in the same subnet ping; neighbors outside it route via a gateway.

Read-only verification commands.

Common mistakes

Subtracting 2 from every prefix — /31 and /32 have different rules.
Mixing up mask and wildcard — a /26 is 255.255.255.192 but wildcard 0.0.0.63.
Counting the network or broadcast address as usable host IPs on normal subnets.
Computing the network by rounding to the nearest .0 boundary of the wrong block size — always derive block size from the mask first.

Two hosts that 'should' reach each other cannot

Observation: Host A 10.1.1.70/26 and Host B 10.1.1.140/26 cannot ping each other directly.

Hypotheses

They are in different subnets despite similar addresses
Missing gateway configuration
ACL blocking traffic

Check	Indicates
Recompute each subnet	/26 blocks are .0/.64/.128/.192 — .70 is in .64–.127, .140 is in .128–.191: different subnets.
Gateway addresses on each host	Each needs a gateway inside its own block to route between subnets.

Correction: Put both hosts in the same subnet or route between them via a router interface in each subnet.

Traffic flows — via the gateway when subnets differ, directly when they match.

On this page

The subnetting method
/31, /32 and wildcards
Verification and mistakes

Related resources

Practise this topic Source topic 13: Subnetting (Part 1)Practise topic 13 deck Source topic 14: Subnetting (Part 2)

Sources

Cisco CCNA 200-301 v1.1 Exam Topics (official PDF) — reviewed 2026-11-02
RFC 3021 — Using 31-Bit Prefixes on IPv4 Point-to-Point Links — reviewed 2026-11-02
RFC 4632 — Classless Inter-domain Routing (CIDR) — reviewed 2026-11-02
Cisco — IP Addressing Services Command Reference — reviewed 2026-11-02

Targets CCNA 200-301 v1.1 objectives. Supplemental blocks are labeled beyond exam scope.