2 cissp ® Official Study Guide Eighth Edition

Network Layer Protocols and IP Networking Basics
Another important protocol in the TCP/IP protocol suite operates at the Network layer of 
the OSI model, namely, Internet Protocol (IP). IP provides route addressing for data pack-
ets. It is this route addressing that is the foundation of global internet communications 
because it provides a means of identity and prescribes transmission paths. Similar to UDP, 
IP is connectionless and is an unreliable datagram service. IP does not offer guarantees that 
packets will be delivered or that packets will be delivered in the correct order, and it does 
not guarantee that packets will be delivered only once. Thus, you must employ TCP on IP 
to gain reliable and controlled communication sessions.

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TCP/IP Model 
459
IPv4 vs. IPv6
IPv4 is the version of Internet Protocol that is most widely used around the world. How-
ever, a version known as 
IPv6
is being adopted both for private and public network use. 
IPv4 uses a 32-bit addressing scheme, while IPv6 uses 128 bits for addressing. IPv6 
offers many new features that are not available in IPv4. Some of IPv6’s new features 
are scoped addresses, autoconfiguration, and Quality of Service (QoS) priority values. 
Scoped addresses give administrators the ability to group and then block or allow access 
to network services, such as file servers or printing. Autoconfiguration removes the need 
for both Dynamic Host Configuration Protocol (DHCP) and Network Address Translation 
(NAT). QoS priority values allow for traffic management based on prioritized content.
IPv6 is supported by most operating systems released since 2000, either natively or via an 
add-in. However, IPv6 has been slowly adopted. Most of the IPv6 networks are currently 
located in private networks such as those in large corporations, research laboratories, and 
universities. For a glimpse into the status of IPv4 to IPv6 conversion on the internet, see 
the IPv6 statistics at 
https://www.google.com/intl/en/ipv6/statistics.html
.
IP classes
Basic knowledge of IP addressing and IP classes is a must for any security professional. If 
you are rusty on addressing, subnetting, classes, and other related topics, take the time to 
refresh yourself. Table 11.3 and Table 11.4 provide a quick overview of the key details of 
classes and default subnets. A full Class A subnet supports 16,777,214 hosts; a full class B 
subnet supports 65,534 hosts; and a full Class C subnet supports 254 hosts. Class D is used 
for multicasting, while Class E is reserved for future use.
TA b l e 11. 3
IP classes