When you’re talking about blocking IP ranges in a firewall (like iptables, nftables, or CSF on AlmaLinux/CentOS/RHEL), the biggest block you can apply is the entire IPv4 or IPv6 address space, depending on what you want to achieve.
IPv4
-
The largest CIDR block possible is:
This matches all IPv4 addresses (about 4.3 billion).
-
In practice, admins usually block in smaller chunks (e.g.,
/8
,/16
,/24
) to avoid unintentionally cutting off legitimate traffic. -
Example of blocking an entire
/8
(16.7 million IPs):
IPv6
-
The largest block is:
This covers all IPv6 addresses (a mind-boggling number, ~3.4×10³⁸).
-
Similar logic applies: you can block entire subnets, like
/32
or/48
depending on the target range.
Practical Guidance
-
Use specific ranges whenever possible. Blocking
0.0.0.0/0
or::/0
is essentially shutting down your server’s networking. -
Common large blocks:
-
/8
(huge, often too big unless you’re cutting out an entire provider or country) -
/16
(65,536 IPs, good for ISP ranges) -
/24
(256 IPs, typical for small networks)
-
???? So technically, the biggest IP range you can block is /0
(all IPs), but practically the largest useful range is usually /8
in IPv4 or /32
–/48
in IPv6.
Here is a simple graph showing available IPs within a network:
CIDR |
Subnet Mask |
Total IPs |
Usable IPs |
/32 | 255.255.255.255 | 1 | 1 |
/31 | 255.255.255.254 | 2 | 0 |
/30 | 255.255.255.252 | 4 | 2 |
/29 | 255.255.255.248 | 8 | 6 |
/28 | 255.255.255.240 | 16 | 14 |
/27 | 255.255.255.224 | 32 | 30 |
/26 | 255.255.255.192 | 64 | 62 |
/25 | 255.255.255.128 | 128 | 126 |
/24 | 255.255.255.0 | 256 | 254 |
/23 | 255.255.254.0 | 512 | 510 |
/22 | 255.255.252.0 | 1024 | 1022 |
/21 | 255.255.248.0 | 2048 | 2046 |
/20 | 255.255.240.0 | 4096 | 4094 |
/19 | 255.255.224.0 | 8192 | 8190 |
/18 | 255.255.192.0 | 16,384 | 16,382 |
/17 | 255.255.128.0 | 32,768 | 32,766 |
/16 | 255.255.0.0 | 65,536 | 65,534 |
/15 | 255.254.0.0 | 131,072 | 131,070 |
/14 | 255.252.0.0 | 262,144 | 262,142 |
/13 | 255.248.0.0 | 524,288 | 524,286 |
/12 | 255.240.0.0 | 1,048,576 | 1,048,574 |
/11 | 255.224.0.0 | 2,097,152 | 2,097,150 |
/10 | 255.192.0.0 | 4,194,304 | 4,194,302 |
/9 | 255.128.0.0 | 8,388,608 | 8,388,606 |
/8 | 255.0.0.0 | 16,777,216 | 16,777,214 |