CTI 2500-C100, Инструкция по установке и эксплуатации

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CTI 2500 Installation and Operation Guide V2.8 100
Subnet Mask
Used alone, the designation of network classes proved to be inflexible. Assigning large
numbers of devices to the same network is impractical considering performance, topology,
and security constraints. The Subnet Mask, often called the Network Mask, provides a
flexible means of designating the Network ID portion and Host ID portions of the network
address. In modern TCP/IP implementations, the network class is largely ignored, except for
setting the default subnet mask
The subnet mask is a collection of 32 bits that distinguish the network ID portion of the IP
address from the host ID. Subnet masks are implemented by assigning 1's to bits that
belong to the network ID and 0's to the bits that belong to the host ID. To represent the
mask, the 32-bit value is converted to dotted decimal notation or CIDR (Classless Inter-
Domain Routing) notation, a commonly used alternate. The CIDR notation counts the
number of bits in Network ID portion of the address (bits that are set to 1). The count is
preceded by a slash. See the example below.
Bits for Network Mask Dotted Decimal Format CIDR Format
1111111
1 00000000 00000000 00000000 255.0.0.0 /8
1111111
1 11111111 11110000 00000000 255.255.240.0 /20
1111111
1 11111111 11111111 00000000 255.255.255.0 /24
For example: when the IP address is 128.54.177.97 and the network mask is
255.255.255.0, the Network ID is 128.54.177 and the Host ID is 97. When using the CIDR
format, the network mask is appended to the IP address. Thus the IP address and subnet
mask shown above would be represented as 128.54.177.97 /24.
NOTE
The binary representation of a Network Mask must be a single continuous block 1’s followed by a
contiguous block of zeroes. When entering the Network Mask in dotted decimal notation, you must
ensure that this requirement is maintained. For example, a network mask of 255.247.0.0 is not valid
because the binary equivalent (11111111111101110000000000000000) violates this rule.
The Network Mask must allow at least two bits of host address. In addition, a network mask
which causes the derived host ID to be 0 or a broadcast address (all Host ID bits set to 1)
should not be used.
Selecting IP Addresses
If your network administrator has assigned IP addresses for the network you are using, you
should obtain an IP address from the administrator.
If you are establishing your own IP addresses, you should select addresses from a block of
‘private” addresses established by the Internet Assigned Numbers Authority (IANA).