If router is a layer 3 device how can it have an application interface (GUI)?

Question

It is said that layer 3 devices such as routers do not have an application layer. But whenever I want to configure my home router I punch in the IP address and open up its graphical user interface. What should we call that type of "applications" of such low level devices?

"Importantly, you do not need to write software that runs on network-core devices, such as routers or link-layer switches. Even if you wanted to write application software for these network-core devices, you wouldn’t be able to do so." Computer networking a top down approach by James Kurose

Answer 1

It is said that layer 3 devices such as routers do not have an application layer.

No, routers route at layer-3, not the application layer, but that does not mean a router cannot have a user interface in the application layer.

But whenever I want to configure my home router I punch in the IP address and open up its graphical user interface.

Home networking and consumer-grade devices are explicitly off-topic here.

What should we call that type of "applications" of such low level devices?

Such a user interface is an application. A PC is also a layer-3 device, but it can run applications. A router is also a host, like a PC, and, in fact, a routing protocol (exchanges routing information between routers, but does not actually route) is an application.

"Importantly, you do not need to write software that runs on network-core devices, such as routers or link-layer switches. Even if you wanted to write application software for these network-core devices, you wouldn’t be able to do so." Computer networking a top down approach by James Kurose

That is because most enterprise-grade devices run a proprietary OS for which you have no way to write an application, although the manufacturer can and does write applications for its devices. This is changing a bit, e.g. Cisco has some OSes for which you can write applications under certain circumstances with certain restrictions, but the router process is run as a separate, proprietary process.

Answer 2

You should make a difference between router function and a router appliance.

The router function is located in the network layer. A router receives packets and forwards them between networks based on their destination addresses, according to its routing table.

A router appliance's main function is routing. Additionally, it may support all kinds of additional functions on various layers, including and not limited to

• network address translation
• filter based on network (L3) or transport layer (L4) information
• track L4 connection states for stateful filtering
• VPN
• switching
• web server, ssh, telnet or serial console for configuration
• DHCP server, relay, snooping (with switching)
• authentication, authorization and accounting (AAA)
• SSL deep inspection
• malware scanning
• intrusion prevention
• intrusion detection

Appliances with a lot of features are often marketed as firewall, UTM appliance or various other names, but most also include a router.

Answer 3

Just to muddy the waters even further, the routing function ( forwarding packets) happens at layer 3. But many routing protocols, like BGP and RIP, run in the application layer. OSPF and EIGRP are applications too (sort of) but they don’t quite fit into OSI or TCPIP model.

Answer 4

Importantly, you do not need to write software that runs on network-core devices, such as >routers or link-layer switches. Even if you wanted to write application software for these >network-core devices, you wouldn’t be able to do so."

Even these lines are being blurred today. There are some companies with "software" routers and they are providing functionality, SDKs, etc. for developers to write application software for and on top of existing router and firewall functionality. These companies are mostly API-driven interfaces but still have a GUI for those who want it.