What a /4 subnet is: the multicast block
A /4 fixes only the first four bits. Its one well-known use is 224.0.0.0/4, the entire IPv4 multicast range.
At a glance
- Subnet mask
- 240.0.0.0
- Wildcard mask
- 15.255.255.255
- Total addresses
- 268,435,456
- Usable hosts
- 268,435,454
- Contains
- 1,048,576 networks of /24
- Host bits
- 28 of 32
In detail
Four fixed bits leave 28 free, so a /4 holds 268,435,456 addresses, one sixteenth of the address space. Nobody allocates a /4 to an organisation. The prefix exists in practice to name whole classes of address at once.
224.0.0.0/4 is multicast, the old class D. 240.0.0.0/4 is the reserved former class E, which is still unusable on the public internet despite periodic proposals to reclaim it.
If a host is sending to something in 224.0.0.0/4, it is doing multicast, and the usual questions are whether IGMP snooping is on and whether the switch is flooding or pruning the group.
Examples
224.0.0.0/4 multicast, addresses 224.0.0.0 to 239.255.255.255
240.0.0.0/4 reserved (former class E), 240.0.0.0 to 255.255.255.255
# a link-local multicast group inside that range
224.0.0.251 mDNSThings that catch people out
- 224.0.0.0/24 inside the multicast block is link-local and is never forwarded by routers, whatever the time to live says.
- 255.255.255.255 sits inside 240.0.0.0/4 but is the limited broadcast address, not a reserved unicast address.
Try it: IP Subnet Calculator
Enter any IPv4 address with a prefix or a dotted mask and get the network address, broadcast address, wildcard mask, usable host range, address class, and the hex and binary forms.
IP Subnet Calculator
LOCAL · Browser-OnlyCalculate IP subnet details, network ranges, and CIDR notation
192.168.1.0192.168.1.255255.255.255.00.0.0.255192.168.1.0/24192.168.1.1 - 192.168.1.254192.168.1.1192.168.1.254256254C0xC0A8016411000000.10101000.00000001.01100100Split into two /25 subnets
Dividing 192.168.1.0/24 one prefix deeper.
Common Subnet Masks
Related IPv4 subnets
All ipv4 subnets →What a /8 subnet is: a full class A block
A /8 uses the mask 255.0.0.0 and gives 16,777,214 usable addresses. What it is for and where it goes wrong.
What a /0 subnet is: the default route
A /0 uses the mask 0.0.0.0 and gives 4,294,967,294 usable addresses. What it is for and where it goes wrong.
What a /24 subnet is: the classic LAN subnet
A /24 uses the mask 255.255.255.0 and gives 254 usable addresses. What it is for and where it goes wrong.
What a /10 subnet is: the carrier-grade NAT block
A /10 uses the mask 255.192.0.0 and gives 4,194,302 usable addresses. What it is for and where it goes wrong.
What a /12 subnet is: the 172.16 private block
A /12 uses the mask 255.240.0.0 and gives 1,048,574 usable addresses. What it is for and where it goes wrong.
What a /14 subnet is: a four-way /16 aggregate
A /14 uses the mask 255.252.0.0 and gives 262,142 usable addresses. What it is for and where it goes wrong.
Tools for this
- IP Subnet Calculator Free online IP subnet calculator - calculate IP subnet details, CIDR notation, and network address ranges
- IP Address Lookup Free online IP address lookup - find your public IP address and basic network information
- Number Base Converter Free online number base converter - convert numbers between decimal, binary, octal, and hexadecimal