Glossary · Networking
Subnet (CIDR block)
A subnet is a contiguous block of IP addresses that share the same leading bits, written in CIDR notation as an address, a slash and the number of fixed bits — 198.51.100.0/24, for example, is 256 IPv4 addresses — as defined for IPv4 in RFC 4632.
Reviewed 30 Oct 2026
import ipaddressfrom collections import Counterexits = ["198.51.100.23", "198.51.100.180", "203.0.113.7"]nets = Counter(ipaddress.ip_network(f"{ip}/24", strict=False) for ip in exits)for net, n in nets.items(): print(net, n) # 198.51.100.0/24 2, 203.0.113.0/24 1Reading CIDR notation
The number after the slash counts the fixed leading bits. In IPv4 a prefix of length n holds 2^(32−n) addresses. CIDR replaced the old class A, B and C networks, whose size was implied by the first bits of the address, with prefixes of any length.
| Prefix | Addresses | Old name |
|---|---|---|
| /32 | 1 | A single host |
| /24 | 256 | Class C size |
| /16 | 65,536 | Class B size |
| /8 | 16,777,216 | Class A size |
| IPv6 /64 | 2⁶⁴ | One IPv6 network segment |
Why subnets matter for proxies
- Sites act on ranges. Blocks and rate limits are often applied to a whole prefix, not one address, so ten exits from one /24 can behave like one.
- Ranges map to networks. Prefixes are what an ASN announces to the internet, so a range lookup also tells a site who runs the network.
- Diversity is measured in ranges. For spreading load, exits spread over many prefixes and networks count for more than a large number of addresses in one block — a point that matters most with IPv6 proxies.
Working with subnets in code
Python's standard ipaddress module answers the common questions — which network an exit belongs to, and whether two exits share one — without any external lookup. Pair it with the checks in test proxy location when you audit a set of exits.
FAQ
Questions
Something not covered? Write to contact@arcproxies.io.
How many IP addresses are in a /24?
256. A /24 fixes the first 24 of 32 bits, leaving 8 bits, and 2⁸ is 256. Conventionally the first and last are reserved on a classic LAN, but the block itself holds 256 addresses.
What is the difference between a subnet and an ASN?
A subnet is a block of addresses; an ASN identifies the network that announces one or more such blocks to the internet. One ASN usually covers many prefixes.
Does subnet diversity matter for web scraping?
It matters wherever a site limits or blocks by range. Exits spread across many prefixes and networks spread per-range counters; many exits from one block do not.
Can I choose the subnet of my exit on ARC?
No. ARC targeting is by pool, country and city. The pool decides the type of network, and the gateway assigns exits within it.
Sources & review
Last reviewed by the ARC Proxies team. Prices are read from the live catalog at build time.
In practice