Cisco PIX 520 - PIX Firewall 520 Online Help Manual page 251

Mask, Netmask, IP Subnet Mask
A mask is a 32-bit field which shows how an Internet address is to be divided into network, subnet and host parts.
The netmask has ones in the bit positions in the 32-bit address which are to be used for the network and subnet
parts, and zeros for the host part. The mask should contain at least the standard network portion (as determined by
the address's class), and the subnet field should be contiguous with the network portion.
Netmask Basics
How Subnet Masks are Used to Determine the Network Number. The router performs a set process to determine
the network (or more specifically, the subnetwork) address. First, the router extracts the IP destination address
from the incoming packet and retrieves the internal subnet mask. It then performs a logical AND operation to
obtain the network number. This causes the host portion of the IP destination address to be removed, while the
destination network number remains. The router then looks up the destination network number and matches it
with an outgoing interface. Finally, it forwards the frame to the destination IP address. Specifics regarding the
logical AND operation are discussed in the following section.
Logical AND Operation. Three basic rules govern logically "ANDing" two binary numbers. First, 1 "ANDed"
with 1 yields 1. Second, 1 "ANDed" with 0 yields 0. Finally, 0 "ANDed" with 0 yields 0. Two simple guidelines
exist for remembering logical AND operations: Logically "ANDing" a 1 with a 1 yields the original value, and
logically "ANDing" a 0 with any number yields 0.
The following figure illustrates that when a logical AND of the destination IP address and the subnet mask is
performed, the subnetwork number remains, which the router uses to forward the packet. Applying a logical AND
the destination IP address and the subnet mask produces the subnetwork number.
Netmask Logical AND
TCP/IP
TCP/IP (Transmission Control Protocol) is a connection-oriented transport layer protocol that provides reliable
full-duplex data transmission in an IP environment. TCP corresponds to the transport layer (Layer 4) of the OSI
reference model. Among the services TCP provides are stream data transfer, reliability, efficient flow control,
full-duplex operation, and multiplexing.
With stream data transfer, TCP delivers an unstructured stream of bytes identified by sequence numbers. This
service benefits applications because they do not have to chop data into blocks before handing it off to TCP.