IPv4 block sizes are commonly expressed using CIDR notation. A /24 contains 256 total IPv4 addresses, a /23 contains 512, a /22 contains 1,024, a /21 contains 2,048, a /20 contains 4,096 and a /19 contains 8,192 total IPv4 addresses.
Here is the quick reference:
| CIDR | Total IPv4 Addresses |
|---|---|
| /32 | 1 |
| /31 | 2 |
| /30 | 4 |
| /29 | 8 |
| /28 | 16 |
| /27 | 32 |
| /26 | 64 |
| /25 | 128 |
| /24 | 256 |
| /23 | 512 |
| /22 | 1,024 |
| /21 | 2,048 |
| /20 | 4,096 |
| /19 | 8,192 |
| /18 | 16,384 |
| /17 | 32,768 |
| /16 | 65,536 |
What Does /24 Mean?
The “/24” is CIDR notation.
CIDR stands for Classless Inter-Domain Routing.
IPv4 addresses contain 32 bits.
The number following the slash tells us how many of those bits identify the network prefix.
Therefore:
/24 leaves 8 bits for addresses.
2⁸ = 256
So a /24 contains 256 total IPv4 addresses.
How Many IP Addresses Are in a /23?
A /23 contains:
512 total IPv4 addresses.
A /23 is equivalent in size to two contiguous /24 networks.
For businesses requiring several hundred public addresses, a /23 can be a useful allocation size. IPv4 leasing at this size is common.
How Many IP Addresses Are in a /22?
A /22 contains:
1,024 total IPv4 addresses.
A /22 is equivalent in size to:
Two /23s
or
Four /24s.
Organizations with larger infrastructure requirements may consider a /22 when a /23 does not provide sufficient capacity.
How Many IP Addresses Are in a /21?
A /21 contains:
2,048 total IPv4 addresses.
A /21 contains the same total address capacity as:
- 2 × /22
- 4 × /23
- 8 × /24
A /21 represents a significant public IPv4 allocation and should generally be deployed with careful routing, utilization and reputation management.
How Many IP Addresses Are in a /20?
A /20 contains:
4,096 total IPv4 addresses.
That’s equivalent to:
- 2 × /21
- 4 × /22
- 8 × /23
- 16 × /24
At this size, IPv4 address space represents a substantial network resource.
How Many IP Addresses Are in a /19?
A /19 contains:
8,192 total IPv4 addresses.
A /19 can be divided into:
- 2 × /20
- 4 × /21
- 8 × /22
- 16 × /23
- 32 × /24
This illustrates why CIDR notation is useful. Large allocations can be subdivided into smaller contiguous networks according to operational requirements.
IPv4 CIDR Block Size Formula
IPv4 contains 32 bits.
To calculate the total number of addresses in a CIDR block, use:
2^(32 - prefix length)
Example for a /22:
32 - 22 = 10
2¹⁰ = 1,024
Therefore a /22 contains 1,024 total addresses.
Example for a /19:
32 - 19 = 13
2¹³ = 8,192
Therefore a /19 contains 8,192 total addresses.
Total Addresses vs. Usable Addresses
There is an important distinction between the total number of addresses contained in a CIDR block and the number available to individual devices in a particular subnet design.
In traditional IPv4 subnetting, the first address is generally used as the network address and the last as the broadcast address.
For example, a traditionally configured /24 subnet contains:
256 total addresses
254 conventionally usable host addresses
However, modern routed infrastructure does not always assign an entire public IPv4 block as one traditional Layer 2 subnet.
Providers can route address space using architectures where the traditional “subtract two” assumption does not describe how the entire allocation is utilized.
Therefore, when leasing or deploying public IPv4 space, discuss the actual routing architecture with the network provider rather than assuming that every deployment will use conventional subnetting.
Which IPv4 Block Size Do I Need?
The correct allocation depends upon the number of addresses required today and anticipated future growth.
As a simple reference:
But don’t select an allocation solely because the math works.
Consider:
- Actual utilization
- Growth requirements
- Routing architecture
- Bandwidth
- Server infrastructure
- BGP requirements
- Network reputation
- Provider requirements
Can IPv4 Blocks Be Divided?
Yes.
CIDR blocks can be divided into smaller networks.
For example:
One /21 can become two /22s.
One /22 can become two /23s.
One /23 can become two /24s.
This can be useful when separating customers, applications, locations or network functions.
Why Contiguous IPv4 Space Matters
A contiguous block is generally easier to route and manage than hundreds of unrelated individual addresses.
For example, rather than maintaining 512 unrelated IP addresses, a network operator can manage a single /23 prefix.
Contiguous allocations can simplify:
- Routing
- Firewall policies
- Documentation
- Reverse DNS
- Monitoring
- Network administration
- Reputation management
IPv4 Blocks and BGP
Larger IPv4 deployments often involve BGP.
BGP—Border Gateway Protocol—is the routing protocol used by autonomous networks to exchange information about how Internet prefixes can be reached.
Businesses deploying dedicated IPv4 infrastructure should determine whether:
- The provider will originate the prefix
- The customer's ASN will originate it
- An LOA is required
- RPKI configuration is involved
- The provider will manage routing
Learn more about BGP.
Leasing an IPv4 Block
Businesses do not necessarily have to purchase IPv4 address space permanently.
IPv4 blocks can also be leased.
Depending upon availability and technical requirements, NetWest can evaluate business requirements for allocations such as:
/24, /23, /22, /21 and other configurations.
NetWest can also combine IPv4 resources with routing, bandwidth, colocation and network engineering.