換算為秒
1分	1時	1日	1週(7Day)	1月(30Day)	1年(365Day)
60	3600	86400	604800	2592000	31536000

RFC1918 name	Network #	IP address range	Broadcast	number of addresses	largest CIDR block (subnet mask)	host id size	mask bits	classful description [Note 1]
24-bit block	10.0.0.0	10.0.0.1 – 10.255.255.254	10.255.255.255	16777216	10.0.0.0/8 (255.0.0.0)	24 bits	8 bits	single class A network
20-bit block	172.16.0.0	172.16.0.1 – 172.31.255.254	172.31.255.255	1048576	172.16.0.0/12 (255.240.0.0)	20 bits	12 bits	16 contiguous class B networks
16-bit block	192.168.0.0	192.168.0.1 – 192.168.255.254	192.168.255.255	65536	192.168.0.0/16 (255.255.0.0)	16 bits	16 bits	256 contiguous class C networks

Subnet Mask Cheat Sheet

	Addresses	Hosts	Netmask	Amount of a Class C
/30	4	2	255.255.255.252	1/64
/29	8	6	255.255.255.248	1/32
/28	16	14	255.255.255.240	1/16
/27	32	30	255.255.255.224	1/8
/26	64	62	255.255.255.192	1/4
/25	128	126	255.255.255.128	1/2
	Addresses	Hosts	Netmask	Amount of a Class B
/24	256	254	255.255.255.0	1
/23	512	510	255.255.254.0	2
/22	1024	1022	255.255.252.0	4
/21	2048	2046	255.255.248.0	8
/20	4096	4094	255.255.240.0	16
/19	8192	8190	255.255.224.0	32
/18	16384	16382	255.255.192.0	64
/17	32768	32766	255.255.128.0	128
/16	65536	65534	255.255.0.0	256
	Addresses	Hosts	Netmask	Amount of a Class A
/15	131072	131070	255.254.0.0	512
/14	262144	262142	255.252.0.0	1024
/13	524288	524286	255.248.0.0	2048
/12	1048576	1048574	255.240.0.0	4096
/11	2097152	2097150	255.224.0.0	8192
/10	4194304	4194302	255.192.0.0	16384
/9	8388608	8388606	255.128.0.0	32768
/8	16777216	16777214	255.0.0.0	65536

Guide to sub-class C blocks

/25 -- 2 Subnets -- 126 Hosts/Subnet

Network #	IP Range	Broadcast
.0	.1-.126	.127
.128	.129-.254	.255

/30 -- 64 Subnets -- 2 Hosts/Subnet

/26 -- 4 Subnets -- 62 Hosts/Subnet

/27 -- 8 Subnets -- 30 Hosts/Subnet

/28 -- 16 Subnets -- 14 Hosts/Subnet

/29 -- 32 Subnets -- 6 Hosts/Subnet