6 S&C Instruction Sheet 1074-510
Operation
Subnet Masking
Applying a subnet mask to an IP address allows the
network and node parts of the address to be identified.
The network bits are represented by the “1”s in the mask,
and the node bits are represented by the “0”s. Performing
a bitwise logical
And
operation between the IP address
and the subnet mask results in the Network Address or
Number. The Network Address is also called the subnet.
For example, using the test IP address and the default
Class B subnet mask, we get:
10101100.00011010.11110000.11001000 172.26.240.200
Class B IP address
11111111.11111111.00000000.00000000 255.255.000.000
Default Class B subnet mask
10101100.00011010.00000000.00000000 172.26.000.000
network address
Default subnet masks:
• Class A–255.0.0.0–
11111111.00000000.00000000.00000000
• Class B–255.255.0.0–
11111111.11111111.00000000.00000000
• Class C–255.255.255.0–
11111111.11111111.11111111.00000000
Private Subnets
Three IP network addresses are reserved for private
networks. The addresses are 10.0.0.0, Subnet Mask 255.0.0.0;
172.16.0.0, Subnet Mask 255.240.0.0; and 192.168.0.0, Subnet
Mask 255.255.0.0. These addresses are also notated
10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16. They can be used
by anyone setting up internal IP networks, such as a lab or
home LAN behind a NAT or proxy server or a router. It is
always safe to use these because routers on the Internet
by default will never forward packets coming from these
addresses.
Subnetting an IP network can be done for a variety of
reasons, including organization, use of different physical
media (such as Ethernet, FDDI, WAN, etc.), preservation
of address space, and security. The most common reason
is to control network traffic. In a traditional unswitched
Ethernet network, all nodes on a segment see all the
packets transmitted by all the other nodes on that segment.
Performance can be adversely affected under heavy traffic
loads, because of collisions, and the resulting retransmis-
sions. A router is used to connect IP networks to minimize
the amount of traffic each segment must receive.
MAC Address
In networking, the Media Access Control (MAC) address
is a unique identifier programmed into each network
device. This number acts like a name for the device, and
all SpeedNet ME radios have unique MAC addresses.
Some devices have user-configurable MAC addresses, but
the SpeedNet ME radio MAC address is configured at the
factory and cannot be changed. Most protocols use MAC
addresses that are globally unique, but not all protocols
use MAC addresses or require they be unique.
Unlike IP addresses, MAC addresses do not have node
and network sections, and a receiving node cannot deter-
mine any network information from the MAC address. The
length of a MAC address is six bytes, and an IP address
is four bytes long. Therefore, the MAC address cannot be
represented using an IP address. So, an IP address must
be mapped to its corresponding MAC address. Address
Resolution Protocol (ARP) is used to locate a specific MAC
address. ARP broadcasts an ARP request packet, which
contains the source MAC address, the source IP address,
and the destination IP address. Each node in the local
network receives this packet. A node that has the speci-
fied destination IP address returns an ARP reply packet
containing its MAC address to the originating host.
On broadcast networks, such as Ethernet, the MAC
address allows each node to be uniquely identified and
allows frames to be marked for specific nodes. It thus
forms the basis of most of the layer 2 networking upon
which higher OSI layer protocols are built to produce
complex functioning networks. See Table 1 on page 7.
