IPv4 and Routing Glossary

Terms used across this documentation, defined the way they are used operationally rather than the way a textbook defines them. Abbreviations appear next to the full term.

These definitions also back the inline term tooltips elsewhere in the docs, so a term marked up on another page shows the same wording you see here.

A

Address plan

A documented scheme for dividing address space into aggregates and assignments, usually hierarchical so that routes summarise cleanly at each boundary.

Aggregation

Advertising several contiguous prefixes as one shorter prefix to reduce table size. Requires the covered space to be contiguous and under common routing policy.

Anycast

Announcing the same prefix from multiple locations so that routing delivers each client to the topologically nearest instance.

Autonomous System(AS)

A network with a single, clearly defined external routing policy, identified by an AS number and operated by one administrative entity.

Autonomous System Number(ASN)

A 16- or 32-bit identifier assigned by an RIR that identifies an autonomous system in BGP. Required to announce your own address space.

B

Blocklist

A published list of addresses or prefixes associated with abuse. Listing affects mail delivery and access to some services regardless of who currently holds the block.

Bogon

A prefix that should never appear in the global routing table — unallocated, reserved, or private space. Bogon filters at the edge silently drop such announcements.

Broadcast address

The all-ones host address of an IPv4 subnet. Not assignable to a host, which is why a /24 offers 254 usable addresses rather than 256.

C

Carrier-Grade NAT(CGNAT)

NAT operated by a provider that shares one public address across many subscribers, typically using RFC 6598 space (100.64.0.0/10) on the inside.

CIDR

Classless Inter-Domain Routing. Notation and allocation scheme that expresses a prefix as an address plus a prefix length, replacing the fixed class A/B/C boundaries.

CIDR block

A contiguous range of addresses described by a single prefix, aligned so that its first address is a multiple of its size.

D

Default-free zone(DFZ)

The set of routers that carry a full BGP table and have no default route. Whether your prefix is visible in the DFZ is the practical test of global reachability.

Dual stack

Running IPv4 and IPv6 simultaneously on the same infrastructure, so hosts reach each destination over whichever protocol both ends support.

G

Geofeed

A CSV file published by the address holder that maps prefixes to locations, defined in RFC 8805 and referenced from registry records under RFC 9632. Geolocation providers ingest it to correct their databases.

H

Hijack

An announcement of address space by an AS not authorised to originate it, whether accidental or deliberate. RPKI origin validation is the main structural defence.

Host address

An address within a subnet that can be assigned to an interface, excluding the network and broadcast addresses in conventional subnetting.

I

Inetnum

The RIPE database object that records an IPv4 range, its holder, and its status. Its accuracy is what an RIR audit checks.

Internet Routing Registry(IRR)

A distributed set of databases holding route and AS objects used to build prefix filters. Many transit providers will not accept an announcement without a matching route object.

IP Address Management(IPAM)

The practice and tooling for planning, assigning, and auditing address space, typically integrated with DNS and DHCP records.

L

LACNIC

The regional internet registry for Latin America and the Caribbean.

Legacy space

Address space allocated before the RIR system existed and not covered by a standard registry agreement. Transfer rules for it differ by region.

Letter of Authorization(LOA)

A signed statement from the address holder permitting a named party to announce a prefix. Upstreams request it before accepting an announcement of space registered to someone else.

Local Internet Registry(LIR)

An organisation that holds a membership agreement with an RIR and assigns address space to its own customers and infrastructure.

Longest prefix match

The forwarding rule that selects the most specific matching route. It is why a more specific announcement overrides a covering aggregate, and why hijacks often use /24s.

N

Netmask

A 32-bit value marking which leading bits of an address identify the network. Equivalent to a prefix length: 255.255.255.0 is /24.

Network address

The all-zeros host address of a subnet, used to identify the subnet itself rather than a host within it.

Network Address Translation(NAT)

Rewriting addresses in packet headers as they cross a boundary, generally to let many private hosts share fewer public addresses.

P

Prefix

A block of addresses written as a base address and a length, such as 198.51.100.0/24. Shorter prefixes cover more addresses.

Prefix length

The number of leading bits fixed in a prefix. Each additional bit halves the block: a /23 is two /24s.

Provider-independent space(PI)

Address space assigned directly to an end user rather than through a provider, so it can be kept when changing upstreams.

R

RADb

One of the widely used internet routing registries, commonly used to publish route objects for prefixes when the RIR database is not authoritative for them.

RDAP

Registration Data Access Protocol. The structured, JSON-based successor to WHOIS, with consistent responses and defined authentication.

Regional Internet Registry(RIR)

One of five bodies (RIPE NCC, ARIN, APNIC, LACNIC, AFRINIC) that allocate address space and AS numbers within a defined service region.

Reverse DNS(rDNS)

Mapping an address back to a name through PTR records in the in-addr.arpa zone. Mail receivers commonly reject senders without consistent forward and reverse records.

RIPE NCC

The regional internet registry for Europe, the Middle East, and parts of Central Asia.

Route object

An IRR record binding a prefix to the AS number authorised to originate it. Used to generate upstream prefix filters.

Route Origin Authorization(ROA)

A cryptographically signed RPKI object stating which AS may originate a prefix and up to what length. Missing or stale ROAs cause announcements to be treated as invalid.

Route Origin Validation(ROV)

Checking a received BGP announcement against published ROAs and marking it valid, invalid, or not-found. Most large networks now drop invalids.

RPKI

Resource Public Key Infrastructure. A certificate hierarchy that lets address holders make verifiable statements about who may announce their space.

S

Subnet

A division of a larger block into a smaller routable or assignable unit, made by extending the prefix length.

Subnetting

Splitting a block into equal-sized smaller blocks by borrowing bits from the host portion of the address.

Supernet

A block that covers several smaller prefixes, used when announcing or documenting them as one aggregate.

T

Transfer

The registry process that moves the holdership of address space from one organisation to another. Rules, required justification, and holding periods vary by RIR.

U

Usable addresses

The addresses in a subnet assignable to hosts, conventionally the block size minus the network and broadcast addresses. Point-to-point /31 links are the common exception.

Utilization

The proportion of held address space recorded as assigned. RIRs use it to judge requests for additional space and to check that registry records reflect actual use.

V

Variable Length Subnet Mask(VLSM)

Subnetting a block into pieces of differing sizes so each segment gets only what it needs, rather than splitting everything into equal parts.

W

WHOIS

The long-standing query protocol for registration data. Its free-text responses differ between registries, which is why RDAP is replacing it for automation.

Wildcard mask

The bitwise inverse of a netmask, used in access control lists on some platforms where a set bit means “ignore this bit”.