I am new to Cisco and I am studing for the CCNA exam. I am slightly confused on when to use the 2^?-2 and when not to. I found this on the Cisco website, "On the issue of using subnet zero and the all-ones subnet, RFC 1878 states, "This practice (of excluding all-zeros and all-ones subnets) is obsolete. Modern software will be able to utilize all definable networks." Today, the use of subnet zero and the all-ones subnet is generally accepted and most vendors support their use. However, on certain networks, particularly the ones using legacy software, the use of subnet zero and the all-ones subnet can lead to problems."
But several of the study books say use the 2^? -2 to find subnets. Can anyone clear up when I am suppose to use the -2 and when not to.