Classful vs classless routing, ip classless, ICMP redirects, directed broadcasts and IP unnumbered

Classful versus classless routing protocols

In the early internet, the first bits of an address defined its class and therefore its mask: class A /8, class B /16, class C /24 (see IPv4 addressing). Classful routing protocols such as RIP version 1 and IGRP relied on this. Their updates carried only the network address, not the mask, and the receiving router guessed the mask from the class or from its own interface in the same major network.

This breaks two common designs. VLSM (variable-length subnet masks), such as /30 links alongside /24 LANs in 10.0.0.0/8, cannot be described without masks. Discontiguous networks, where subnets of one major network sit on both sides of a different network, fail because each side summarises to the same classful network, for example both advertising 172.16.0.0/16, and the router in the middle cannot tell which side holds which subnet.

Classless protocols include the mask with every route, so any prefix length works. Watch for auto-summary, a classful habit that older EIGRP and RIPv2 software turned on by default. Disable it so subnets are advertised as they are:

router rip
 version 2
 no auto-summary
router eigrp 100
 no auto-summary

The ip classless command

Classful ideas also affected forwarding. Suppose a router knows 10.1.1.0/24 and 10.1.2.0/24, and has a default route. A packet arrives for 10.1.9.5. With no ip classless, the router reasons that it knows the major network 10.0.0.0 and should therefore know every subnet of it, so it drops the packet instead of using the default route. With ip classless, it simply uses the longest match, which here is the default route. ip classless has been the default for many years, so you only meet the old behaviour in exams and very old configurations.

ICMP redirects

If a router receives a packet and must send it back out of the same interface towards a next hop in the sender's own subnet, there was a better path. The router forwards the packet and sends the host an ICMP redirect telling it to use the other router directly. This helps poorly configured hosts, but redirects can also be forged to divert traffic, and they cost CPU. Many hardening guides disable them on user-facing interfaces, and they are usually disabled automatically where HSRP is used.

interface GigabitEthernet0/0
 no ip redirects
 no ip unreachables
 no ip proxy-arp

Proxy ARP, another feature often disabled here, is explained in ARP explained.

IP directed broadcasts and IP unnumbered

A directed broadcast is a packet sent to the broadcast address of a remote subnet, such as 192.168.20.255. If the last router converted it into a local broadcast, one packet from outside could reach every host in the subnet. Attackers abused this in "smurf" amplification attacks, so routers have dropped directed broadcasts by default since the late 1990s (no ip directed-broadcast). If you need it, for example to send Wake-on-LAN packets from a management server, enable it only on the target interface and restrict it with an ACL so only that server can use it.

IP unnumbered lets a point-to-point interface borrow the address of another interface, normally a loopback, instead of using its own subnet. It saves addresses on serial and tunnel links and is used with PPPoE and dial-up style virtual interfaces. The trade-offs are that you cannot ping or monitor the link by its own address, and it only suits point-to-point links.

interface Loopback0
 ip address 10.255.0.1 255.255.255.255
interface Serial0/1/0
 ip unnumbered Loopback0

Real-world example: the forgotten auto-summary

A retail chain added a new branch with LAN 172.16.50.0/24, connected over a 10.0.0.0/30 link to head office, which already had 172.16.10.0/24. The branch router ran old EIGRP software with auto-summary on, so it advertised 172.16.0.0/16 to head office, and head office did the same in return. Each router discarded the other's summary because it pointed to its own network, and the two LANs could not reach each other. Adding no auto-summary on both routers made them advertise the real /24 subnets and fixed the problem at once.