S&C Instruction Sheet 1074-510
7
Description
Subnet Masking
Applying a subnet mask to an IP address allows you to identify the network and node
parts of the address. 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 our 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
There are three IP network addresses 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 organiza-
tion, use of different physical media (such as Ethernet, FDDI, WAN, etc.), preservation
of address space, and security. Th
e 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, due to collisions and the resulting retransmissions. 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 that they be unique.
Unlike IP addresses, MAC addresses do not have node and network sections, and a
receiving node cannot determine any network information from the MAC address. The
length of a MAC address is 6 bytes and an IP address is 4 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 pac
ket, 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 specified destination IP
addres
s returns an ARP reply packet containing its MAC address to the o
riginating 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 8.
