Question 1

What is CIDR notation and why do ISPs use it?

Accepted Answer

CIDR (Classless Inter-Domain Routing) notation represents an IP address and its routing prefix using a slash followed by the prefix length, such as 192.168.1.0/24. It replaced the old class-based system (Class A, B, C) in 1993, allowing ISPs to allocate address space more efficiently. Instead of being locked into fixed /8, /16, or /24 blocks, ISPs can assign exactly the right-sized subnet for each purpose — a /30 for an OLT uplink, a /24 for a subscriber VLAN, or a /20 for a NAT pool.

Question 2

How should ISPs plan their subnet allocation?

Accepted Answer

ISPs should plan subnets based on function and growth projections. Common practice: use /30 or /31 for point-to-point links between routers and OLTs, /24 to /22 for subscriber segments, /28 to /26 for management VLANs, and /20 or larger for CGNAT pools. Always leave room for growth — it is much easier to allocate a slightly larger subnet upfront than to renumber later. Document all allocations in an IPAM system and use monitoring to track utilization.

Question 3

What is the difference between a subnet mask and a wildcard mask?

Accepted Answer

A subnet mask uses 1-bits to identify the network portion and 0-bits for the host portion (e.g., 255.255.255.0 for a /24). A wildcard mask is the inverse — it uses 0-bits for the network and 1-bits for the host portion (e.g., 0.0.0.255). Wildcard masks are used in router access control lists (ACLs) and OSPF configurations. The relationship is simple: wildcard = 255.255.255.255 minus the subnet mask.

Question 4

What is CGNAT and why do ISPs need it?

Accepted Answer

Carrier-Grade NAT (CGNAT) allows ISPs to share a single public IPv4 address among multiple subscribers using the 100.64.0.0/10 shared address space defined in RFC 6598. With IPv4 addresses nearly exhausted, CGNAT lets ISPs continue growing their subscriber base without purchasing expensive public IPv4 blocks. The trade-off is increased NAT complexity and potential issues with services that require direct inbound connections. ISPs typically allocate /20 or larger subnets from the 100.64.0.0/10 range for CGNAT pools.

Question 5

Can I use /31 subnets for point-to-point links?

Accepted Answer

Yes. RFC 3021 allows /31 subnets for point-to-point links, providing exactly 2 usable addresses without wasting IPs on network and broadcast addresses. This is standard practice for router-to-router and router-to-OLT interconnects. Most modern network equipment supports /31 subnets. If you are using older equipment that does not support /31, use /30 instead, which provides 2 usable addresses out of 4 total.

Question 6

How do I monitor subnet utilization across my ISP network?

Accepted Answer

Effective subnet monitoring requires tracking IP address utilization per VLAN and subnet, DHCP pool exhaustion, ARP table sizes on routers and switches, and interface traffic levels. Network monitoring tools like NetSense NMS provide switch and port monitoring that helps ISPs identify oversubscribed segments before they cause connectivity issues. Combining subnet planning with real-time monitoring ensures efficient address space usage and prevents exhaustion.

CIDR	Subnet Mask	Hosts	Typical ISP Use
/32	255.255.255.255	1	Loopback, management IP
/31	255.255.255.254	2	Point-to-point links (RFC 3021)
/30	255.255.255.252	2	OLT uplinks, router interconnects
/29	255.255.255.248	6	Small customer allocation
/28	255.255.255.240	14	Small office, CPE management
/27	255.255.255.224	30	Medium customer block
/26	255.255.255.192	62	OLT management VLAN
/25	255.255.255.128	126	Large customer block
/24	255.255.255.0	254	Standard LAN, subscriber segment
/22	255.255.252.0	1,022	Large subscriber pool
/20	255.255.240.0	4,094	NAT pool, cloud VPC
/16	255.255.0.0	65,534	Large ISP internal network
/8	255.0.0.0	16.8M	Massive network (Class A)

CIDR	Range	Addresses	Common Use
10.0.0.0/8	10.0.0.0 – 10.255.255.255	16.7M	Cloud VPCs, large ISP backbones
172.16.0.0/12	172.16.0.0 – 172.31.255.255	1M	Docker, medium networks
192.168.0.0/16	192.168.0.0 – 192.168.255.255	65K	Home networks, CPE management
100.64.0.0/10	100.64.0.0 – 100.127.255.255	4.2M	CGNAT shared address space (RFC 6598)

CIDR	Subnet Mask	Hosts	Typical ISP Use
/32	255.255.255.255	1	Loopback, management IP
/31	255.255.255.254	2	Point-to-point links (RFC 3021)
/30	255.255.255.252	2	OLT uplinks, router interconnects
/29	255.255.255.248	6	Small customer allocation
/28	255.255.255.240	14	Small office, CPE management
/27	255.255.255.224	30	Medium customer block
/26	255.255.255.192	62	OLT management VLAN
/25	255.255.255.128	126	Large customer block
/24	255.255.255.0	254	Standard LAN, subscriber segment
/22	255.255.252.0	1,022	Large subscriber pool
/20	255.255.240.0	4,094	NAT pool, cloud VPC
/16	255.255.0.0	65,534	Large ISP internal network
/8	255.0.0.0	16.8M	Massive network (Class A)

IPv4 Subnet Calculator

Subnet Mask Cheat Sheet

Private & Reserved IP Ranges

Subnet Mask Cheat Sheet (192.168.1.0/24 default)

Frequently Asked Questions