Juniper IGP - CONFIGURATION GUIDE V11.1.X, Руководство по настройке

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NOTE: The router supports a 31-bit mask on point-to-point links. This means that
the IP address 1.2.3.4 255.255.255.254 is valid. A point-to-point link in which only
one end supports the use of 31-bit prefixes may not operate correctly.
Figure 3: Basic Network Masking
Classes A, B, and C have the following natural masks, which define the network and
host portions of each class:
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Class A natural mask 255.0.0.
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Class B natural mask 255.255.0.0
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Class C natural mask 255.255.255.0
The use of masks can divide networks into subnetworks by extending the network
portion of the address into the host portion. Subnetting increases the number of
subnetworks and reduces the number of hosts.
For example, a network of the form 10.0.0.0 accommodates one physical segment
with about 16 million hosts on it. Figure 4 on page 10 shows how the mask
255.255.0.0. is applied to network 10.0.0.0. The mask divides the IP address 10.0.0.1
into a network portion of 10, a subnet portion of 0, and a host portion of 0.1. The
mask has borrowed a portion of the host space and has applied it to the network
space. The network space of the class 10 has increased from a single network 10.0.0.0
to 256 subnetworks, ranging from 10.0.0.0 to 10.255.0.0. This process decreases
the number of hosts per subnet from 16,777,216 to 65,536.
Figure 4: Subnetting
Classless Addressing with CIDR
Classless interdomain routing (CIDR) is a system of addressing that improves the
scaling factor of routing in the Internet. CIDR does not use an implicit mask based
on the class of network. In CIDR, an IP network is represented by a prefix, which is
an IP address and an indication of the leftmost contiguous significant bits within this
address.
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IP Addressing
JUNOSe 11.0.x IP, IPv6, and IGP Configuration Guide