Card 09/ 10
Practice: Dividing a /16 Between Three Teams
Three teams need 500, 200 and 60 hosts; give each a block that fits without overlapping the others.
Dividing a range between several needs is the subnet skill used most in practice, whenever a network is planned. Your company has 10.20.0.0/16. Three teams each need their own subnet, with room for at least the number of hosts below, counting the two reserved addresses in each block but nothing extra:
- Team A: 500 hosts
- Team B: 200 hosts
- Team C: 60 hosts
Give each team the smallest block that fits, place the three blocks so that none overlaps another, and write each as CIDR with its first and last address.
Before you check, say why it helps to place the largest block first.
Show one plan that works
| Team | Needs | Prefix | Usable hosts | CIDR block | First to last |
|---|---|---|---|---|---|
| A | 500 | /23 | 510 | 10.20.0.0/23 | 10.20.0.0 to 10.20.1.255 |
| B | 200 | /24 | 254 | 10.20.2.0/24 | 10.20.2.0 to 10.20.2.255 |
| C | 60 | /26 | 62 | 10.20.3.0/26 | 10.20.3.0 to 10.20.3.63 |
Each prefix is the longest that still fits. A /24 has 254 usable hosts, too few for 500, so Team A needs a /23. A /25 has 126, too few for 200, so Team B needs a /24. A /26 has 62, just enough for 60.
Placing the largest block first keeps every block on its natural boundary: a /23 must start on an even third byte, a /24 on any third byte, a /26 at .0, .64, .128 or .192. Laid out from largest to smallest, each block starts exactly where the last one ended. Other plans are correct too, as long as each block starts on its own boundary and no two share an address.
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