Foundation project
Project: plan an office network
Lesson 8 of 20 · 18 min · Module 2: Python’s ipaddress toolkit
A new office needs separate networks for guests, staff, and printers, each sized to fit. Plan the allocation automatically, like a network engineer using VLSM (variable-length subnet masks).
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By the end, you’ll be able to…
- Size a subnet for a number of hosts
- Allocate subnets in a safe order
- Detect when a plan does not fit
The idea
Give the largest needs the first, largest subnets. For a team that needs n hosts, the smallest prefix that fits is 32 - (n + 2 - 1).bit_length(): room for n devices plus the network and broadcast addresses. Allocating largest first keeps every subnet aligned.
for hosts in (10, 50, 100):
print(hosts, 32 - (hosts + 1).bit_length())01From hosts to prefix
50 hosts need 52 addresses. The next power of two is 64 = 2^6, so the subnet needs 6 host bits: a /26. (50 + 1).bit_length() is 6, so the prefix is 32 - 6 = 26.
print((50 + 1).bit_length())602Largest first keeps alignment
A /26 must start at a multiple of 64. Placing the biggest subnets first means every later, smaller subnet starts on a boundary that suits it.
03Track a cursor
Keep the next free address as an integer. After each allocation, add the subnet size. If the last address of a new subnet would pass the end of the base network, the plan does not fit.
Your project brief
plan_subnets(base, needs). base is an IPv4 CIDR string and needs maps a team name to the number of hosts it needs (at least 1). Allocate subnets from the start of base, largest need first (ties by team name), each as small as possible. Return a dict mapping each team to its CIDR string. Raise ValueError if the teams do not fit.What the checks look for
- Plans guests, staff and printers in a /24
- Uses the smallest subnet that fits exactly
- Breaks ties by team name
- Raises ValueError when the teams do not fit
Take this with you
Size by hosts + 2, allocate largest first, and check the remaining space every time.
Go a little further
Return the unused space at the end as a list of free CIDR blocks.