Page 767 - (ISC)² CISSP Certified Information Systems Security Professional Official Study Guide

Class Default subnet mask CIDR equivalent

A 255.0.0.0 /8

B 255.255.0.0 /16

C 255.255.255.0 /24

Note that the entire Class A network of 127 was set aside for the

loopback address, although only a single address is actually needed for
that purpose.

Another option for subnetting is to use Classless Inter-Domain
Routing (CIDR) notation. CIDR uses mask bits rather than a full
dotted-decimal notation subnet mask. Thus, instead of 255.255.0.0, a
CIDR is added to the IP address after a slash, as in 172.16.1.1/16, for
example. One significant benefit of CIDR over traditional subnet-

masking techniques is the ability to combine multiple noncontiguous
sets of addresses into a single subnet. For example, it is possible to
combine several Class C subnets into a single larger subnet grouping.
If CIDR piques your interest, see the CIDR article on Wikipedia or visit
the IETF’s RFC for CIDR at http://tools.ietf.org/html/rfc4632.

ICMP and IGMP are other protocols in the Network layer of the OSI
model:

ICMP Internet Control Message Protocol (ICMP) is used to

determine the health of a network or a specific link. ICMP is utilized
by ping, traceroute, pathping, and other network management tools.
The ping utility employs ICMP echo packets and bounces them off
remote systems. Thus, you can use ping to determine whether the
remote system is online, whether the remote system is responding

promptly, whether the intermediary systems are supporting
communications, and the level of performance efficiency at which the
intermediary systems are communicating. The ping utility includes a
redirect function that allows the echo responses to be sent to a
different destination than the system of origin.

Unfortunately, the features of ICMP were often exploited in various
forms of bandwidth-based denial-of-service (DoS) attacks, (DoS), such

as ping of death, smurf attacks, and ping floods. This fact has shaped
how networks handle ICMP traffic today, resulting in many networks