This question already has an answer here:
- How does IPv4 Subnetting Work? 5 answers
Assuming CIDR and classless addressing, the CCNA books I'm reading say
- Pearson 640-822 official cert guide 2012, pgs 403-404: According to their method
S = P - N(subnet, prefix, host bits respectively). In this case
S = 8 - 24 < 0.
- CCNA guide 7th literally says in its subnettting chapter that the prefixes
/15can only be used with class A addresses.
So for example, a subnet 18.104.22.168 cannot have a net mask of 255.0.0.0. Does that really make sense and hold true in the real world? Can a network or subnet really be assigned an IP address with any prefix?
I'm practicing on a c2691 router and it takes the
ip route 22.214.171.124 255.0.0.0 Serial 0/0 command no problem. If 126.96.36.199 is a subnet, then what is its network?
It also takes
ip route 192.0.0.0 192.0.0.0 Serial 0/0. Here we have a "class C" subnet with a mask actually less than
255.0.0.0! Is that possible in the real world?
I just tried adding some routes to my Cisco router and the following output is possible. You can assign the same IP
188.8.131.52 more than once in the same route table. Interestingly, it seems that it won't say "subnetted" unless the route you added has mask at least
R2(config)#do show ip route ... 184.108.40.206/16 is subnetted, 1 subnets S 220.127.116.11 [1/0] via 192.168.1.1 S* 18.104.22.168/24 [1/0] via 192.168.1.1 10.0.0.0/24 is subnetted, 3 subnets O 10.1.6.0 [110/128] via 10.1.4.2, 04:54:56, Serial0/1 C 10.1.5.0 is directly connected, Serial0/0 C 10.1.4.0 is directly connected, Serial0/1 C 192.168.1.0/24 is directly connected, FastEthernet0/0 S 22.214.171.124/7 [1/0] via 192.168.1.1 S 126.96.36.199/7 [1/0] via 192.168.1.1 S 188.8.131.52/6 [1/0] via 192.168.1.1 S 184.108.40.206/6 [1/0] via 192.168.1.1 S 220.127.116.11/6 [1/0] via 192.168.1.1