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Classful routing and all zero subnets

Добавлено: 29 мар 2017 13:27
Юра Руденко(yurmag)
Читаю Routing TCP/IP и вот парочкастранных предложений:

"classful protocols, cannot differentiate between an all-zero subnet and the major network number" and "subnet bits cannot be all zeros or all ones in classful environments"

Согласно сказанному, если дробить сеть 172.16.0.0/16 на куски по /26:
- 172.16.1.0/26

- 172.16.1.64/26
- 172.16.1.128/26

- 172.16.1.192/26

То первый и последний кусок использовать не рекомендуется.

однако у Циски есть дока - Subnet Zero and the All-Ones Subnet, в которой говорится:

"Traditionally, it was strongly recommended that subnet zero and the all-ones subnet not be used for addressing. According to RFC 950 leavingcisco.com, "It is useful to preserve and extend the interpretation of these special (network and broadcast) addresses in subnetted networks. This means the values of all zeros and all ones in the subnet field should not be assigned to actual (physical) subnets."

В той же доке:

"It should be noted that even though it was discouraged, the entire address space including subnet zero and the all-ones subnet have always been usable. The use of the all-ones subnet was explicitly allowed and the use of subnet zero is explicitly allowed since Cisco IOS Software Release 12.0."

и

"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subnet zero and the all-ones subnet, RFC 1878 states, "This practice (of excluding all-zeros and all-ones subnets) is obsolete.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."


Кто в теме почему изначально возникла рекомендация не использовать zero subnets?