Network, Broadcast, and Host Ranges

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Plain-English explanation

Every subnet has a network address (all host bits 0) and a broadcast address (all host bits 1). Neither is assigned to a host in classic IPv4. Usable hosts sit between them.

Given 192.168.10.45/26:

Block size is 64
Network for that block: 192.168.10.0
Broadcast: 192.168.10.63
First usable: 192.168.10.1
Last usable: 192.168.10.62
Usable count: 62

Deep dive

Finding the block: Divide the interesting octet by block size; the quotient × block size = network octet value.

For 10.1.1.50/25:

Block 128 → networks 10.1.1.0 and 10.1.1.128
50 < 128 → block starts at .0
Broadcast: 10.1.1.127
Usable: 10.1.1.1 – 10.1.1.126

Usable host formula (classic): 2^host_bits - 2

| Prefix | Total | Usable (classic) | |--------|-------|------------------| | /24 | 256 | 254 | | /25 | 128 | 126 | | /26 | 64 | 62 | | /27 | 32 | 30 | | /28 | 16 | 14 | | /29 | 8 | 6 | | /30 | 4 | 2 |

Step-by-step — is 172.16.5.200/21 a usable host?

/21 → mask 255.255.248.0, block 8 in third octet
Multiples: … 0, 8, 16 … 248 — 5 is in 0–7? Wait: third octet is 5, block starts at 0 for 172.16.0.0/21
Actually /21 on 172.16.5.200: third octet block size 8, network 172.16.0.0, broadcast 172.16.7.255
172.16.5.200 is a valid host in that range

Practice until this feels automatic.

Commands to know

Cisco — check interface in subnet

show ip interface GigabitEthernet0/0 | include Internet show ip route connected

Troubleshooting

| Error | Consequence | |-------|-------------| | Gateway set to network address | Hosts cannot route | | Gateway set to broadcast | ARP anomalies, failures | | Two subnets overlap | Unpredictable routing | | Wrong broadcast assumed | ACL or OSPF network mismatch |

Convention: gateway is often first usable (.1) — not a protocol rule, but expect it in labs.

Exam relevance

/31 and /32 note

Point-to-point links may use /31 with two usable endpoints in modern IOS (RFC 3021). For most CCNA subnetting questions, use classic network/broadcast rules unless the question specifies /31 P2P.

Exam trap

"How many hosts" vs "how many usable hosts" — read carefully. /29 total is 8; usable classic is 6.

Practice checklist

Solve five range problems in under 30 seconds each
For each /24–/29, recite network/broadcast pattern without calculator
Identify whether a given IP is network, broadcast, or usable
Explain why network and broadcast cannot be assigned to hosts (classic model)
Pass timed drills on the subnetting trainer range mode

What is the broadcast address for 10.1.1.50/25?

How many usable host addresses in a /28 subnet (classic rules)?