# Terraform cidrsubnet
Terraform 에 cidrsubnet 이라는 함수가 있는데, 설명을 읽어도 이해가 안되서,
삽질을 좀 하며 이해한 내용을 정리합니다.
더불어 ipcalc 라는 완소 유틸리티도 사용해보세요.
cidrsubnet("10.0.0.0/20", <newbit>, <newnum>)
10.0.0.0/20 =
❯ ipcalc 10.0.0.0/20
Address: 10.0.0.0 00001010.00000000.0000 0000.00000000
Netmask: 255.255.240.0 = 20 11111111.11111111.1111 0000.00000000
Wildcard: 0.0.15.255 00000000.00000000.0000 1111.11111111
=>
Network: 10.0.0.0/20 00001010.00000000.0000 0000.00000000
HostMin: 10.0.0.1 00001010.00000000.0000 0000.00000001
HostMax: 10.0.15.254 00001010.00000000.0000 1111.11111110
Broadcast: 10.0.15.255 00001010.00000000.0000 1111.11111111
Hosts/Net: 4094 Class A, Private Internet
이걸 2^<newbit> 만큼 쪼갠 후, 그 중 <newnum> 번 째 블럭을 가르키는 cidrblock 표현식을 리턴
ex:
cidrsubnet("10.0.0.0/20", 2, 1)
-> 10.0.0.0/20 을 2^2 = 4 개로 쪼갬.
10.0.0.0/22
❯ ipcalc 10.0.0.0/22
Address: 10.0.0.0 00001010.00000000.000000 00.00000000
Netmask: 255.255.252.0 = 22 11111111.11111111.111111 00.00000000
Wildcard: 0.0.3.255 00000000.00000000.000000 11.11111111
=>
Network: 10.0.0.0/22 00001010.00000000.000000 00.00000000
HostMin: 10.0.0.1 00001010.00000000.000000 00.00000001
HostMax: 10.0.3.254 00001010.00000000.000000 11.11111110
Broadcast: 10.0.3.255 00001010.00000000.000000 11.11111111
Hosts/Net: 1022 Class A, Private Internet
10.0.4.0/22
❯ ipcalc 10.0.4.0/22
Address: 10.0.4.0 00001010.00000000.000001 00.00000000
Netmask: 255.255.252.0 = 22 11111111.11111111.111111 00.00000000
Wildcard: 0.0.3.255 00000000.00000000.000000 11.11111111
=>
Network: 10.0.4.0/22 00001010.00000000.000001 00.00000000
HostMin: 10.0.4.1 00001010.00000000.000001 00.00000001
HostMax: 10.0.7.254 00001010.00000000.000001 11.11111110
Broadcast: 10.0.7.255 00001010.00000000.000001 11.11111111
Hosts/Net: 1022 Class A, Private Internet
10.0.8.0/22
❯ ipcalc 10.0.8.0/22
Address: 10.0.8.0 00001010.00000000.000010 00.00000000
Netmask: 255.255.252.0 = 22 11111111.11111111.111111 00.00000000
Wildcard: 0.0.3.255 00000000.00000000.000000 11.11111111
=>
Network: 10.0.8.0/22 00001010.00000000.000010 00.00000000
HostMin: 10.0.8.1 00001010.00000000.000010 00.00000001
HostMax: 10.0.11.254 00001010.00000000.000010 11.11111110
Broadcast: 10.0.11.255 00001010.00000000.000010 11.11111111
Hosts/Net: 1022 Class A, Private Internet
10.0.12.0/22
❯ ipcalc 10.0.12.0/22
Address: 10.0.12.0 00001010.00000000.000011 00.00000000
Netmask: 255.255.252.0 = 22 11111111.11111111.111111 00.00000000
Wildcard: 0.0.3.255 00000000.00000000.000000 11.11111111
=>
Network: 10.0.12.0/22 00001010.00000000.000011 00.00000000
HostMin: 10.0.12.1 00001010.00000000.000011 00.00000001
HostMax: 10.0.15.254 00001010.00000000.000011 11.11111110
Broadcast: 10.0.15.255 00001010.00000000.000011 11.11111111
Hosts/Net: 1022 Class A, Private Internet
이렇게 4개로 쪼개짐.
--------
그 중 첫번째 블럭인 10.0.0.0/22 가 리턴됨.