CIRCLE WITH A DOT

The more important issue here is that the Internet has changed a lot since our first edition. While the Internet Protocol was invented to allow interconnection of different network technologies and the networks of different organizations, its success has meant that IP is often just used to forward packets among homogeneous links inside a single organization. That doesn't change the meaning of "router", it just illustrates one of many ways the Internet has changed.

The adoption of BGP for routing inside datacenters is another example of how different the Internet and its protocols are today.

This is the challenge we face in writing this new edition: to be correct and precise in our descriptions of technology while admitting that the Internet of today is different than the one we described in our first edition. And to do that in a way that leaves the reader understanding the technology without being overwhelmed by the complexity of today's networks.

More in the newsletter:

https://systemsapproach.org/2026/04/20/not-your-fathers-internet/
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This exposed two issues: first, is there a difference between an L3 switch and a router? And second, when modern switches can be configured to forward on either the Ethernet header or the IP header, how do we best talk about them?

On the first question, we ultimately decided to let context be the determining factor. Even while there is no technical difference between an L3 switch and a router, the most common usage in the datacenter context favors "switch". For ISPs, router is still the preferred term.

The configurability of modern devices, especially those based on programmable forwarding silicon, means we have to be clear in our discussions to separate what a device is capable of doing versus what it is configured to do.

3/n

Here are two images by the two authors representing different views of the naming issues:

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We had a spirited debate in recent weeks regarding the right way to refer to modern networking devices, particularly those found in datacenters. One of us (Bruce) argued that, if it forwards IP packets, it's a router. Larry pushed back: shouldn't a router interconnect heterogeneous networks, or sit on the border between different autonomous systems?

Further confounding the discussion is conflicting usage by industry players: most networking devices in datacenters, even those forwarding IP packets, are called switches. Put a similar device inside a service provider's backbone? Most people call those things routers—so what's the rule for when to use each term?

This might seem like a trivial thing, but when writing a textbook, it's important that we use terminology precisely and consistently if we want our readers to understand the technology fully. Being inconsistent with standard industry practice would help no-one, even if we believe our stance to be "correct". The good news is that we resolved the issue to our satisfaction, and along the way we gained some insights into how much the Internet has changed since we wrote our first edition in 1995. More in this week's newsletter:

Not Your Father’s Internet - Systems Approach

A debate about the difference between a switch and a router highlights the significant changes that have happened to the Internet

Systems Approach (systemsapproach.org)

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Our latest piece on QUIC has appeared in The Register:
https://www.theregister.com/2026/04/16/quic_explained/?td=rt-3a