SkyPass Solutions
Knowledge base

Networking, explained

Plain-language answers on BGP, peering, IP transit, IXPs and network security, written by people who run AS202520.

Glossary

What is BGP?

BGP is the routing protocol of the internet, exchanging routes between autonomous systems. Learn eBGP, iBGP, AS paths and security.

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What is an ASN?

An ASN is the unique number that identifies an autonomous system on the internet. We cover 16- vs 32-bit, RIR assignment and its role in BGP.

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What is peering?

Peering is the direct exchange of traffic between networks. We cover public vs private peering, the settlement-free model, the role of IXPs and ISP benefits.

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What is IP transit?

IP transit is a paid service that gives access to the whole internet through an upstream provider. We cover the full BGP table, 95th-percentile billing and commits.

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What is a route server?

A route server at an IXP enables multilateral peering through a single BGP session. We explain how it works, why it stays out of the AS_PATH and how it filters routes.

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What is multihoming?

Multihoming means connecting a network to the internet through several providers at once. We cover requirements (ASN, PI), benefits, traffic steering and load-balancing myths.

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What is a looking glass?

A looking glass is a public, read-only window into an operator's router. It lets you check BGP routes, ping and traceroute from the perspective of a given network.

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What is an IXP?

An IXP (internet exchange point) is shared infrastructure where networks interconnect and exchange traffic locally. We explain who is there and why to join.

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What is anycast?

Anycast is a technique where one IP address is announced from many locations and traffic reaches the nearest. We cover DNS, CDN and anti-DDoS use cases.

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What is CGNAT?

CGNAT (Carrier-Grade NAT) lets ISPs share public IPv4 addresses across many subscribers. We explain how it works, the drawbacks and the link to IPv6.

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What is MANRS?

MANRS is a routing security initiative. Learn its four actions: filtering, anti-spoofing, coordination and global validation with RPKI.

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What is RPKI?

RPKI is a public key infrastructure that cryptographically proves which AS may originate a prefix in BGP and helps operators drop invalid, hijacked routes.

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What is CIDR?

CIDR is a classless way of addressing IP that writes the mask as a prefix length (e.g. /24). We cover notation, aggregation and classless routing.

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What is a subnet?

A subnet is a logical division of an IP network defined by a mask. We cover masks, network and broadcast addresses, CIDR prefixes, and why and how to subnet.

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What is an IP prefix?

An IP prefix is a block of addresses written as a network address plus a mask length (e.g. 192.0.2.0/24). We cover announcing prefixes in BGP and /24 and /48 filters.

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What is NAT?

NAT (Network Address Translation) rewrites IP addresses in packet headers at the network edge. We explain SNAT, DNAT, PAT and the trade-offs.

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What is a default route?

A default route (0.0.0.0/0, ::/0) matches any address when no more specific route exists. We explain the gateway of last resort and choosing a default route or a full BGP table.

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What is the BGP table?

The BGP table is the set of all IP routes a router knows. The global table (DFZ) holds over 950k IPv4 prefixes. Full table versus a default route.

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What is an AS path (AS_PATH)?

AS_PATH is the BGP attribute listing the autonomous system numbers a prefix traversed. It detects loops, shapes best-path selection and enables prepending.

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What is a BGP community?

A BGP community is a tag attached to a route that carries information or triggers a routing action, such as a local-pref change or AS-path prepend.

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What is LOCAL_PREF?

LOCAL_PREF is the BGP attribute that decides outbound path selection within an AS. The higher value wins, so you can prefer peering over transit.

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What is MED (Multi-Exit Discriminator)?

MED (Multi-Exit Discriminator) is a BGP attribute that hints to a neighboring AS which of several links inbound traffic should enter through. Lower MED wins.

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What is a route reflector?

A route reflector is an iBGP router that spreads routes between clients and drops the full-mesh need. We cover clusters, loop prevention and route servers.

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What is BGP route selection?

BGP route selection is the best-path algorithm that picks one route to a prefix from many candidates using a fixed, ordered list of tie-breaker attributes.

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What is a route leak?

A route leak sends BGP routes beyond the agreed policy scope. Learn the six RFC 7908 types, the causes, and prevention with filtering, RPKI and peerlock.

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What is BGP hijacking?

BGP hijacking is the illegitimate announcement of someone else's IP prefix in BGP, rerouting traffic. See real cases and how RPKI ROV and IRR filtering stop it.

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What is RTBH?

RTBH is a BGP technique that remotely drops traffic to an attacked address into Null0, shielding the rest of the network from DDoS. We explain BLACKHOLE and RFC 7999.

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What is BGP FlowSpec?

BGP FlowSpec (RFC 8955) is a BGP extension that distributes granular traffic-filtering rules (address, port, protocol). It is more precise than RTBH.

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What is a DDoS attack?

A DDoS attack exhausts a victim's resources from many sources at once. Learn the anatomy, botnets, amplification and the three classes of attack.

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What is an IRR (Internet Routing Registry)?

An IRR is a distributed routing database where operators publish route, route6 and as-set objects describing who may announce which prefixes in BGP.

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What is a ROA?

A ROA (Route Origin Authorization) is a signed RPKI record stating which AS may originate a prefix and its maximum allowed prefix length (maxLength).

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What is a RIR?

A RIR is a regional internet registry that allocates IP addresses and ASNs within its region. Learn about the five RIRs, IANA's role and how delegation works.

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What is an LIR?

An LIR is a Local Internet Registry, a RIR member (e.g. RIPE NCC) that gets IP and ASN allocations and assigns them to users. We cover its role and sponsoring.

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What is PI address space?

PI address space is provider-independent IP space you can announce through any operator. We explain PI vs PA, portability and its role in multihoming.

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What is a PNI?

PNI (Private Network Interconnect) is a dedicated point-to-point peering link between two networks. We explain how it differs from public peering at an IXP.

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What is a cross-connect?

A cross-connect is a physical cable (fibre or copper) linking two racks inside a data center via the Meet-Me Room. We cover the MMR, media types and uses.

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What is colocation?

Colocation is renting rack space in a data center with power, cooling and connectivity. We cover racks, U units, A/B power feeds and why operators colocate.

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What is a Point of Presence (PoP)?

A PoP (point of presence) is a physical location where an operator's network meets other networks. We explain edge presence and how PoPs reduce latency.

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What is a CDN?

A CDN is a distributed network of edge servers that cache content closer to users. We cover edge caching, anycast and how CDNs peer with ISPs.

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What is a Tier 1 network?

A Tier 1 network reaches the whole internet purely through settlement-free peering, without buying transit. We explain the Tier 1/2/3 split and the DFZ.

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What is latency?

Latency is the time a packet takes to reach its destination. We explain RTT, the components of delay and why peering and shorter paths reduce it.

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What is jitter?

Jitter is the variation in packet delay, the difference in arrival times between packets. Learn its causes, impact on VoIP and gaming, and how to mitigate it.

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What is packet loss?

Packet loss is data packets that fail to reach their destination. We cover causes, the impact on TCP and real-time traffic, and how to diagnose it with MTR.

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What is MTU?

MTU is the largest packet an interface forwards without fragmentation. We cover jumbo frames, fragmentation, Path MTU Discovery and MSS clamping.

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What is MPLS?

MPLS forwards packets by short labels instead of per-hop IP lookups. Learn label switching, L3VPN (RFC 4364), traffic engineering and fast reroute.

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What is a VLAN?

A VLAN is a logical Layer 2 network carved out within shared infrastructure. We cover 802.1Q tagging, segmentation, trunks and access ports.

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What is ECMP?

ECMP (Equal-Cost Multi-Path) is a routing mechanism that spreads traffic across several equal-cost paths. We explain flow hashing and per-flow packet ordering.

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What is BFD?

BFD (RFC 5880) is a lightweight protocol that detects path failures in milliseconds, independent of BGP or OSPF, so the network reconverges fast.

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What is reverse DNS?

Reverse DNS (rDNS) maps an IP address to a hostname via a PTR record in the in-addr.arpa and ip6.arpa zones. See how an ISP delegates rDNS to customers.

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What is DNSSEC?

DNSSEC cryptographically signs DNS zones so a resolver can verify the authenticity and integrity of answers. We cover RRSIG, DS records and the chain of trust.

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What is an SLA?

An SLA is a service level agreement: guaranteed uptime in percent, latency and packet-loss targets, and service credits paid when they are missed.

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What is a NOC (Network Operations Center)?

A NOC (Network Operations Center) is the team that monitors a network 24/7, responds to incidents and outages, and runs escalation by defined procedures.

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What is BCP38?

BCP38 (RFC 2827) recommends filtering traffic with forged source addresses at the network edge. This curbs IP spoofing and amplification DDoS attacks.

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What is uRPF?

uRPF blocks spoofed source addresses by checking each packet's source against the router's FIB. We explain strict, loose and feasible-path modes.

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What is PeeringDB?

PeeringDB is an open database where networks publish their ASN, IX and facility presence and peering policy. We explain the records and why accuracy matters.

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What is a bogon?

A bogon is an IP prefix that should never appear in global routing: private, reserved and unallocated space. We cover martians, fullbogons and filtering.

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What is a SYN flood?

A SYN flood is a DDoS attack that exhausts connection tables with a flood of unfinished TCP handshakes. How it works, why it hurts and how to stop it.

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What is UDP amplification and reflection?

UDP amplification turns a small spoofed query into a huge reply aimed at the victim. DNS, NTP, memcached, CLDAP and traffic reflected off CDNs.

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What is carpet bombing?

Carpet bombing spreads a DDoS attack across hundreds of addresses in a prefix, so per-IP detection thresholds stay silent while the uplink still fails.

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What is a stateful firewall?

A stateful firewall tracks sessions and knows whether a packet belongs to an established connection, so it drops DDoS traffic without a rule per vector.

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What is a scrubbing center?

A scrubbing center separates attack traffic from legitimate traffic and returns the latter. Always-on and on-demand models, GRE, costs and pitfalls.

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Guide

How an IXP works

An IXP is a shared layer-2 fabric where networks peer directly. Learn about the switching fabric, route servers and bilateral vs multilateral peering.

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Anti-DDoS protection explained

Anti-DDoS protection defends networks against volumetric, protocol and application attacks. Detection, scrubbing, RTBH, FlowSpec and anycast.

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IPv6 for ISPs

IPv6 for ISPs: why deploy it, /32 and /48 addressing, dual-stack, 464XLAT and DS-Lite, peering over IPv6, and a practical rollout plan.

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How to get an ASN

You get a public ASN from a RIR (e.g. RIPE NCC) through an LIR. Learn the requirements, application steps, costs, and when you actually need your own ASN.

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How to get IPv4 addresses

Four routes to IPv4 in 2026: the RIPE NCC waiting list, the transfer market, leasing, and IPv6 as the real fix. Real costs and the policy requirements.

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How to choose an IP transit provider

How to choose an IP transit provider: assess peering depth, latency and routing, SLA terms, DDoS protection and 95th-percentile billing.

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How to Prevent BGP Route Leaks and Hijacks

A BGP hijack announces someone else's prefix; a leak propagates routes outside the allowed relationship. Defenses: RPKI ROV, IRR filters, max-prefix, peerlock.

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How to deploy RPKI

A practical guide to deploying RPKI: creating ROAs in the RIR portal, running ROV validation on routers, and safely dropping invalid BGP routes.

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How to join an IXP

Practical steps to join an internet exchange: a PeeringDB profile, ordering a port and LOA, configuring BGP on the router and sessions with route servers.

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How to read a traceroute

A traceroute lists the routers (hops) to a target and their latency. Learn to tell RTT spikes and single-hop loss from real loss, and to spot asymmetric paths.

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BGP traffic engineering: how to shape inbound and outbound traffic

BGP traffic engineering steers packet paths. Learn local-preference, AS-path prepending, MED and communities to shape inbound and outbound traffic.

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How to set up a BGP session

Set up an eBGP session safely, step by step: prefix filters, max-prefix limits, RPKI origin validation, bogon filtering and MANRS routing best practices.

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DDoS protection for an ISP

How an ISP protects subscribers from DDoS: why blackholing is a defeat, what CGNAT breaks and where the traffic filtering belongs.

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Comparison

Peering vs IP Transit

Peering vs IP transit: how they differ, what each reaches, cost models, latency and when an ISP should blend both approaches.

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Remote IXP vs physical presence

Remote IXP versus physical presence at an exchange: CapEx vs OpEx, reach, latency, operational burden and when a remote port makes sense for a smaller ISP.

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IPv4 vs IPv6

IPv4 vs IPv6: 32-bit versus 128-bit address space, header differences, the role of NAT and CGNAT, adoption status and why ISPs run dual-stack.

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eBGP vs iBGP

eBGP connects different autonomous systems, while iBGP distributes routes inside one AS. Compare next-hop, AS-path, TTL and the typical role of each mode.

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BGP vs OSPF: how they differ and where each belongs

BGP is a policy-driven exterior protocol (EGP) between autonomous systems. OSPF is an interior IGP that picks routes from a cost metric. Scale and use cases.

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PNI vs public IXP peering

PNI is a dedicated 1:1 link between two networks; public peering exchanges traffic over a shared IXP fabric. We compare cost, capacity, control and use cases.

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IP Transit vs CDN

IP transit buys full internet reachability over BGP, while a CDN moves content closer to users. We compare the cost model, reach and control of each.

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Static routing vs BGP

Static routing vs BGP: static is hand-configured routes, BGP exchanges routes dynamically. We cover when static suffices and when you need BGP.

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