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- Private IP addresses are non-routable addresses that can only be used for private networking;
they cannot be routed through the public Internet. They were introduced for address allocation
for the communication between systems within an organization that has no intention to connect
to the Internet, as well as to extend the life of the IPv4 address space.
- Below lists the reserved private IP addresses. Note that the masks or these private address ranges
are not the default subnet masks for the IP address classes – /8, /16, /24.
Address Class Reserved Address Space Number of Networks
Class A 10.0.0.0 – 10.255.255.255 .(10.0.0.0/8) 1 Class A network
Class B 172.16.0.0 – 172.31.255.255 .(172.16.0.0/12) 16 Class B networks
Class C 192.168.0.0 – 192.168.255.255 (192.168.0.0/16) 256 Class C networks
Reference: RFC 1918 – Address Allocation for Private Internets.
- Some other special-purpose reserved IP addresses are 127.0.0.1, the loopback address that
allows a host to test whether its TCP/IP stack is operational; and 0.0.0.0 that is used by Cisco
routers to indicate the default route. Network 128.0.0.0, 191.255.0.0, 192.0.0.0, and
223.255.255.0 are also reserved.
Note: In fact, any IP address starting with decimal 127 (127.0.0.0/8) is reserved for loopback.
Note: Ping from Cisco devices to 127.0.0.1 would fail, as the route is not in their routing tables.
IP Subnetting
- Classful addressing uses the subnet masks of standard classes to determine the network and host
portions of an address. IPv4 networks can be divided into smaller networks called subnets.
Subnetting allows the breaking of a large network into a number of smaller networks (subnets)
using subnet masks other than the convention or standard Class A, B, and C subnet masks
(/8, /16, and /24).
- Some benefits of subnetting are:
i) Reduced network traffic and bandwidth utilization, as well as optimized network
performance (as a result of reduced broadcast traffic).
ii) More efficient use and allocation of network addresses.
iii) Simplified administration and management.
- A subnet address is created by borrowing bits from the host portion of an IP address – host bits.
A subnet mask is a 32-bit number that is associated with an IP address. 1 indicates subnet bit
and 0 indicates host bit. The subnet mask indicates the number of bits that have been borrowed
from the host portion for the subnet portion. Subnet mask is also known as prefix length in
classless addressing rule.
- Subnet masks are assigned on end systems for them to know which subnet they resides in by
knowing the network and host portions of an IP address.
- Prefix notation or slash notation indicates the number of binary 1s in a mask with a slash (/).
Ex: 192.168.0.100/24 IP address = 192.168.0.100, subnet mask = 255.255.255.0.
- The largest subnet mask can only be /30 because at least 2 bits are required for host bits.
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