10
S&C Instruction Sheet 1075-510
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: 101
01100.00011010.11110000.11001000 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 organiza- tion,
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 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 SDR
Radios have unique MAC addresses. Some devices have user-configurable MAC addresses,
but the SpeedNet SDR 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 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 specified 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 5 on page 11.
Installation
