How Packet Routing Works Over the Internet: Routers, BGP, ASNs, and Traceroute

If you are trying to understand why an IP address belongs to one provider, why traffic takes a strange path, or how packets move from your network to a remote service, you are asking a routing question.

This guide explains routing in practical terms: destination IP addresses, default gateways, routing tables, next hops, NAT, ASNs, BGP, and traceroute.

What packet routing means

When you load a website, your device does not open one private wire to the server. It sends packets. Each packet carries source and destination addresses, then moves through routers that decide the next hop.

The important part is that routers do not need to know the whole end-to-end story. A router reads the destination IP, checks its routing table, chooses the best matching route it knows about, and forwards the packet to the next router or final network.

That loop is packet routing over the internet.

Can you explain how packet routing is done over the internet?

Yes. The short answer is:

1. DNS turns a hostname into an IP address.
2. Your device creates packets for that destination IP.
3. If the destination is not local, the packets go to your default gateway.
4. Your router or carrier edge forwards them toward an upstream network.
5. Each router looks at the destination IP and chooses a next hop from its routing table.
6. BGP helps networks learn which ASNs can reach which IP ranges.
7. The packets keep moving hop by hop until they reach the destination network.

The return traffic goes through the same kind of process. It may not use the same path back.

Before routing starts: DNS and packets

Before routing even begins, two things usually happen:

1. DNS lookup: When you type a hostname such as www.example.com into your browser, your device needs the IP address behind that name. DNS translates the hostname into an address the network can route.
2. Packetization: The request is split into smaller packets. Each packet gets headers with details such as source IP, destination IP, protocol, TTL, and other control fields.

If you need a deeper DNS refresher before coming back here, read DNS Records Explained.

Figure 1: Initial steps before routing: DNS lookup and packetization.

How packet routing works step by step

Here is the direct version:

1. Your device creates packets for the destination IP address.
2. If the destination is outside your local network, the packets go to the default gateway.
3. Your router sends them toward your ISP or upstream network.
4. Each router checks the destination IP against its routing table.
5. The best matching route points to the next hop.
6. The packet moves from router to router until it reaches the destination network.
7. Return traffic follows the same general process, though it may take a different path.

That is why routing problems can be hard to explain from one side only. The outbound path, return path, DNS answer, NAT state, and BGP route can each affect what you see.

A simple packet path example

Say your laptop opens www.example.com and DNS returns 203.0.113.24.

The packet path usually looks like this:

laptop -> home router -> ISP router -> transit or peering network -> destination ASN -> web server

Your laptop does not know every hop. It only needs to know where to send traffic next. At home, that next hop is usually your router. Inside provider networks, each router repeats the same basic lookup against its forwarding table.

That is why an IP lookup and a traceroute answer different questions. IP lookup tells you about the address and network context. Traceroute shows a partial view of the path packets take at that moment.

Step 1: Leaving Your Local Network

Your device (computer, phone) is connected to a local network (e.g., your home Wi-Fi). It has a private IP address (like 192.168.1.100) only usable within that local network.

• Destination check: Your device checks whether the destination IP address is on the same local network. If it is, the packet can stay local.
• Default gateway: If the destination is outside your local network, your device sends the packet to its configured default gateway. This is usually your home router, such as 192.168.1.1.
• NAT: Your home router performs Network Address Translation. It replaces the packet's source private IP address with the public IP address assigned to your connection, then keeps state so return traffic can reach the right device.

The packets, now addressed with your public IP as the source and the destination server's IP as the destination, are forwarded by your router to your Internet Service Provider (ISP).

Step 2: The router's decision loop

Once packets hit your ISP's network, they encounter the first of potentially many routers on the public internet. Each router performs a fundamental decision-making process:

1. Packet arrival: The router receives a packet on one of its interfaces.
2. Destination IP check: It reads the destination IP address in the packet header.
3. Routing table lookup: It checks the forwarding or routing table.
4. Best match: It chooses the most specific matching route, often called longest prefix match.
5. Forwarding decision: The matching route tells the router which outgoing interface and next-hop address to use.
6. Packet forwarding: The router sends the packet toward the next hop.
7. TTL decrement: The router reduces the packet's TTL. If TTL reaches zero, the packet is discarded instead of looping forever.

This process repeats at every router the packet encounters along its path.

How routing tables choose a next hop

Routing tables are not just lists of single IP addresses. They usually contain prefixes such as:

203.0.113.0/24 via 198.51.100.1
0.0.0.0/0 via 192.0.2.1

When a router sees destination 203.0.113.24, it chooses the most specific matching prefix. A /24 route beats a broad default route like 0.0.0.0/0.

That next-hop decision is local. The router does not ask every other router on the internet what to do. It forwards the packet based on the table it already has.

Figure 2: Simplified Router Decision Process.

Step 3: Building the Maps - Routing Tables and Protocols

How does a router know where to send packets? It learns this information and builds its routing table using routing protocols.

Routing Tables

A routing table typically contains entries like:

Destination Network	Subnet Mask (or Prefix Length)	Next Hop IP Address	Interface	Metric
203.0.113.0	255.255.255.0 (/24)	198.51.100.1	eth1	10
10.0.0.0	255.0.0.0 (/8)	192.168.5.1	eth0	5
0.0.0.0	0.0.0.0 (/0)	203.0.113.1	eth2	100

• Destination network or prefix: the address range the route applies to.
• Next hop: the next router or gateway to send packets to.
• Interface: the connection that leads toward that next hop.
• Metric: a cost or preference value used when more than one route is possible.

Routing Protocols

Routers don't figure out these paths in isolation; they talk to each other using routing protocols.

• Interior Gateway Protocols: used within one network or Autonomous System. OSPF is a common example.
• Exterior Gateway Protocols: used between Autonomous Systems. On the public internet, that usually means BGP.

Step 4: Crossing boundaries between ASNs with BGP

The internet is not one single network. It is a network of networks. These individual networks, managed by ISPs, hosting networks, universities, carriers, and large companies, are called Autonomous Systems (AS). Each AS is assigned a unique ASN (Autonomous System Number).

• BGP peering: routers at the edges of different ASNs exchange routes.
• Prefix announcements: a network advertises the IP ranges it can reach.
• AS path: BGP routes carry the ASNs the route advertisement has passed through.
• Policy: networks can prefer one route over another for cost, reliability, traffic engineering, or business reasons.

If you want the operator-focused version of this section, continue with ASN Lookup Guide.

Figure 3: Simplified autonomous systems and BGP peering.

Where BGP fits and where it does not

BGP does not inspect every packet and choose a brand-new path for it. Routers forward packets from their forwarding tables.

BGP helps build the route information behind those tables. It tells networks which prefixes are reachable through which neighboring ASNs. Inside a provider network, other routing protocols and local policies still decide the internal path.

That distinction matters when you are troubleshooting. A route can look fine in BGP while a local link, firewall, NAT table, DNS answer, or server is still broken.

Step 5: The Hop-by-Hop Relay

Combining these concepts, a packet's route across the internet looks like this:

1. Origin: the packet leaves your router or default gateway.
2. ISP network: your provider forwards it through its own network.
3. BGP edge: a border router sends it toward a neighboring ASN that can reach the destination prefix.
4. Inter-AS hops: the packet may cross several ASNs.
5. Destination ASN: the packet reaches the network that serves the destination IP.
6. Final delivery: internal routing sends it to the right subnet and server.

Step 6: Reaching the Destination Server

Once the packet arrives at the final router connected to the destination server's local network segment, that router delivers the packet directly to the server based on its IP address.

The server's operating system receives the incoming packets, uses the sequence numbers in the TCP headers to reassemble the original data, and processes the request.

What traceroute can and cannot prove

You can often inspect part of the path with:

traceroute example.com

or on Windows:

tracert example.com

Traceroute is useful because it shows hop-by-hop replies from routers along a path. It is not perfect evidence of the full route.

Reasons traceroute can be incomplete:

• routers may rate-limit or ignore traceroute probes
• the return path can differ from the outbound path
• load balancing can show different hops on repeated runs
• firewalls and carrier networks may hide parts of the path
• private or internal hops may not have useful names

Use traceroute as a routing clue, then compare it with IP lookup, ASN lookup, DNS, and application logs.

Why an internet route can change

Packet paths can change without anything being wrong on your device.

Common reasons:

• your ISP changes an upstream provider
• BGP policy prefers a different neighboring ASN
• a link fails and traffic moves to a backup path
• DNS returns a different CDN or edge address
• a VPN, proxy, mobile carrier, or corporate gateway changes the visible path

This is why an IP location result can look odd even when routing is working. Location is a lookup estimate. Routing is the path packets are taking right now.

The return trip

Communication is usually bidirectional. The destination server needs to send data back to you (e.g., the webpage content). It follows the exact same process in reverse:

• It breaks the response data into packets.
• The source IP is now the server's IP, and the destination IP is your public IP address.
• Packets cross routers and ASNs until they reach your ISP or carrier.
• Your router uses the NAT table entry it created earlier to send the traffic back to the right local device.
• Your device reassembles the response.

The short version

Internet routing is packet forwarding at network scale.

DNS gives your device an address to reach. Your device sends packets toward that address. Routers choose the next hop from routing tables. BGP helps separate networks advertise which IP ranges they can reach. NAT may change the visible source address before packets leave a home, office, or carrier network.

That is the practical model to keep in mind when an IP lookup, traceroute, ASN result, or location result looks surprising.

FAQ

How are packets routed over the internet?

Routers read the destination IP address, choose a matching route from a routing table, and forward the packet to the next hop. That repeats until the packet reaches the destination network.

What does a routing table do?

A routing table maps destination prefixes to next hops and interfaces. Routers use the most specific matching prefix they know about.

Is every hop chosen by BGP?

No. BGP helps networks exchange reachability for IP prefixes, especially between ASNs. Actual packet forwarding uses routing and forwarding tables on each router.

Is BGP the same thing as DNS?

No. DNS translates names into technical records such as IP addresses. BGP helps networks decide which paths to use to reach those IP ranges across the public internet.

Why does routing matter in an IP lookup?

Because the ASN and route-origin organization help explain which network is actually announcing the IP range. That is often more useful than location alone when investigating traffic.

Does routing always follow the shortest physical path?

No. Routing decisions are policy-driven and depend on available paths, business relationships, metrics, and network health. The chosen route is not always the geographically shortest route.

Does traceroute show the exact path packets always take?

No. Traceroute is a snapshot. Routers can rate-limit replies, load balancing can change the visible hops, and return traffic may follow a different path.

Where should I go next if I want to identify who owns an IP block?

Read ASN Lookup Guide and How to Find the ISP or Network Owner From an IP Address.