What is ISP?
Internet Service Provider
An ISP (Internet Service Provider) is the network that connects your network to the internet, and carries traffic to and from everywhere else on your behalf.
For a home or small office, the ISP provides the line (fibre, cable, DSL or wireless), gives the router its WAN address, either fixed or by DHCP, and often an IPv6 prefix by DHCPv6 prefix delegation. Everything the office sends to the internet goes to the ISP's router as the next hop, and the ISP routes the replies back to the address it assigned. Because private addresses such as 10.0.0.0/8 can't be routed on the internet, the office router uses NAT so that everything leaves from the ISP-assigned address; a well-run ISP also drops traffic from any source address that isn't yours, a practice called BCP38.
Larger networks connect to several ISPs for redundancy, and may run BGP with them, announcing their own address space so that traffic can arrive over either connection. ISPs that sell connectivity to other networks are called transit providers; networks also exchange traffic directly with each other at an IX to avoid paying transit for it.
When something breaks at the ISP, the symptoms often look local: the link stays up and the ISP's router still answers pings while nothing beyond it works. Health checks that test beyond the first hop, as in Two internet connections on Debian, catch that.
Lessons that use ISP
- Two internet connections on DebianIntermediate · 45 min
Give a Debian office router a second ISP: a routing table per ISP, source rules and connection marks so replies leave the way they came, per-ISP NAT, and a systemd health check that fails over and back, measured with a real outage.
- Two internet connections on VyOSIntermediate · 45 min
Add a second ISP to a VyOS office router and make it actually work: why a backup default route misses most outages, health checks that test beyond the ISP, per-ISP NAT, port forwards through both ISPs, and what happens to open connections, all measured on a real lab.
- FRR on Debian: a BGP and OSPF routerIntermediate · 55 min
Turn a Debian 13 server into a BGP edge router with FRR from apt: two upstream ISPs, import filters and prepending, OSPF to the office core, routes in the Linux kernel, an nftables firewall for the router itself, measured failover, and the reload and write memory traps.
- IPv6 on the Debian routerIntermediate · 40 min
Add IPv6 to a Debian router with systemd-networkd: ask the ISP for a /56 by DHCPv6 prefix delegation, give each VLAN a /64 and announce it with router advertisements, firewall it with nftables (including the ICMPv6 IPv6 can't work without), and see what happens when the ISP changes the prefix.
- IPv6 on VyOS: setup and routingBeginner · 55 min
Number a site from an ISP's /48 on VyOS: WAN and blackhole routes, SLAAC with Router Advertisements, handing a /56 to a downstream router statically and with DHCPv6 prefix delegation, and the forward firewall IPv6 needs because there's no NAT. Every output comes from a real two-router VyOS lab.
- Office gateway: DHCP, DNS and VRRP on VyOSBeginner · 40 min
Two VyOS routers serving one office LAN: VRRP for a gateway that survives a router failure, DHCP with split pools and a printer reservation, and DNS forwarding. Then a real failover, the DNS outage it exposed, and the fix.
- Setting up VyOS with NATBeginner · 45 min
A VyOS router with a NAT interface for a private LAN and a public interface for a routed public subnet: plain routing, source NAT, port forwarding and the forward firewall that NAT isn't. Every output comes from a real VyOS lab.
- Two Debian routers: VRRP and connection syncIntermediate · 45 min
Pair two Debian 13 routers so the office survives one failing: keepalived VRRP for the gateway and public addresses, conntrackd so open connections survive, NAT to a shared address, health checks, and split brain, all measured in a lab.