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Linux is a family of open source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991 by Linus Torvalds. Linux is typically packaged in a Linux distribution (or distro for short).

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How exactly does linux use prefix length assigned to network interface? (lemmy.world)

submitted 8 months ago by akash_rawal@lemmy.world to c/linux@lemmy.ml

20 comments fedilink hide all child comments

I was exploring direct links between machines, and basically failed to break something.

I assigned IP address 192.168.0.1/24 to eth0 in two ways.

A. Adding 192.168.0.1/24 as usual

# ip addr add 192.168.0.1/24 dev eth0
# ping -c 1 192.168.0.2
PING 192.168.0.2 (192.168.0.2) 56(84) bytes of data.
64 bytes from 192.168.0.2: icmp_seq=1 ttl=64 time=0.051 ms

***
192.168.0.2 ping statistics
***
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 0.051/0.051/0.051/0.000 ms
#

B: Adding 192.168.0.1/32 and adding a /24 route

# ip addr add 192.168.0.1/32 dev eth0
# # 192.168.0.2 should not be reachable.
# ping -c 1 192.168.0.2
ping: connect: Network is unreachable
# # But after adding a route, it is.
# ip route add 192.168.0.0/24 dev eth0
# ping -c 1 192.168.0.2
PING 192.168.0.2 (192.168.0.2) 56(84) bytes of data.
64 bytes from 192.168.0.2: icmp_seq=1 ttl=64 time=0.053 ms

***
192.168.0.2 ping statistics
***
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 0.053/0.053/0.053/0.000 ms
#

Does this mean that adding an IP address with prefix is just a shorthand for adding the IP address with /32 prefix and adding a route afterwards? That is, does the prefix length has no meaning and the real work is done by the route entries?

Or is there any functional difference between the two methods?

Here is another case, these two nodes can reach each other via direct connection (no router in between) but don't share a subnet.

Node 1:

# ip addr add 192.168.0.1/24 dev eth0
# ip route add 192.168.1.0/24 dev eth0
# # Finish the config on Node B
# nc 192.168.1.1 8080 &lt;&lt;&lt; "Message from 192.168.0.1"
Response from 192.168.1.1

Node 2:

# ip addr add 192.168.1.1/24 dev eth0
# ip route add 192.168.0.0/24 dev eth0
# # Finish the config on Node A
# nc -l 0.0.0.0 8080 &lt;&lt;&lt; "Response from 192.168.1.1"
Message from 192.168.0.1

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[–] istdaslol@feddit.de 3 points 8 months ago

Third: with your /24 subnet you told your system it has that many address to talk to. With the /32 you told it has none to talk to. With adding a route you gave the additional info „there is another network called … with a subnet of … wich you can talk to“ So your second solution is more or less equivalent but with extra steps. I don’t know how it’s implemented in the backend but it is different as in the second there is no network per default but you add routes to some. In contrast to there is a network and no routing is needed