CIDR計算器

找出能容納N台主機的最小CIDR區塊。高效率規劃IP位址分配。

所需主機數:

如何使用本工具

1
Type the number of hosts you need to address into the 'Hosts needed' field.
2
Read the 'Smallest CIDR fitting' row to get the prefix length (for example /26).
3
Check the 'Total addresses' and 'Usable hosts' rows to confirm the block is big enough.
4
Look at the 'Address space wasted' row to see how many addresses you would leave unused at that prefix.

在規劃網路時，你從主機數量出發，反推出一個CIDR前綴。劃分子網路需要使用2的冪次：要容納50台主機，至少需要64個位址（其中2個保留給網路位址和廣播位址），即對應一個/26。本工具會計算出滿足你主機數量需求的最小CIDR。

Sizing a new office VLAN by entering the expected device count and picking the prefix that leaves a little headroom.
Quickly checking whether a /24 is overkill for a subnet that only needs 30 hosts.
Comparing the wasted-address count across host estimates before you carve up a parent block.
Teaching subnetting students why 50 hosts needs 64 addresses and not 50.
Planning IP allocation for a fleet of IoT devices and confirming the block will not run out as you scale.
Sanity-checking a colleague's subnet proposal by reproducing the host-to-prefix math in seconds.

為什麼要減去2？▼

每個IPv4子網路都會保留第一個位址（網路位址）和最後一個位址（廣播位址）。只有中間的位址才能分配給主機。