谁说子网掩码的1必须连续？关于像255.255.0.255这样的非连续子网掩码

最新推荐文章于 2023-10-10 08:55:15 发布

转载最新推荐文章于 2023-10-10 08:55:15 发布 · 1.3w 阅读

文章标签：

#internet #network #networking #class #百度 #web

通信技术专栏收录该内容

17 篇文章

订阅专栏

本文通过RFC950和RFC1219等标准详细解释了255.255.0.255这样的非连续子网掩码是否合法，并给出了实际应用案例。

百度上搜索的都是说255.255.0.255的掩码不合法，其实这些人没有读过RFC，也就大惊小怪了；RFC950的内容：

    A Class C Network Case (illustrating non-contiguous subnet bits)

      For this case, assume that the requesting host is on class C
      network 192.1.127.0, has address 192.1.127.19, that there is a
      gateway at 192.1.127.50, and that on network an 3-bit subnet field
      is in use (01011000), that is, the address mask is 255.255.255.88.

      The host sends the ICMP request to 255.255.255.255:

      Source address:          192.1.127.19
      Destination address:     255.255.255.255
      Protocol:                ICMP = 1
      Type:                    Address Mask Request = AM1
      Code:                    0
      Mask:                    0
      //19的二进制是00010011;88的二进制是01011000;AND的结果是00010000

      The gateway can then respond directly to the requesting host.

      Source address:          192.1.127.50
      Destination address:     192.1.127.19
      Protocol:                ICMP = 1
      Type:                    Address Mask Reply = AM2
      Code:                    0
      Mask:                    255.255.255.88.
      //50的二进制是00110010;88的二进制是01011000;AND的结果是00010000,所以这两个机器在一个子网内，destination address直接就是192.1.127.19

      In the diskless workstation case the host sends:

      Source address:          0.0.0.0
      Destination address:     255.255.255.255
      Protocol:                ICMP = 1
      Type:                    Address Mask Request = AM1
      Code:                    0
      Mask:                    0

      192.1.127.50 will hear the datagram, and should respond with this
      datagram:

      Source address:          192.1.127.50
      Destination address:     255.255.255.255
      Protocol:                ICMP = 1
      Type:                    Address Mask Reply = AM2
      Code:                    0
      Mask:                    255.255.255.88.
        Read more: http://www.faqs.org/rfcs/rfc950.html#ixzz0VE0qJANw
      另外一篇：
      相信读过TCP-IP详解的都知道这个问题，位于中文版vol 1 004里面
      10. (3.5) Is the subnet mask 255.255.0.255 valid for a class A address?  See
      RFC 1219 [Tsuchiya 1991] for more info.  This is available from the 
      Networking I web page, as well as from the usual sources.
      It's valid and it's called a noncontiguous subnet mask since the 16 bits for
      the subnet mask are not contiguous.  The RFC's, however, recommend against
      using noncontiguous subnet masks.
      //看到没，只是反对使用，没说不连续的子网掩码是非法的
      在RFC 950 - Internet Standard Subnetting Procedure中：
      Since the bits that identify the subnet are specified by a bitmask, they need not be adjacent in the address. However, we recommend that the subnet bits be contiguous and located as the most significant bits of the local address
      RFC-950 [2] specifies a procedure for subnetting Internet addresses using a bit-mask.  While RFC-950 allows the "ones" in the subnet mask to be non-contiguous, RFC-950 recommends that 1) they be contiguous, and 2) that they occupy the most significant bits of the "host" part of the internet address.