Which of the following is an extension to Network Address Translation that permits multiple devices providing services on a local area network (LAN) to be mapped to a single public IP address?
Correct Answer: C
Port Address Translation (PAT), is an extension to network address translation
(NAT) that permits multiple devices on a local area network (LAN) to be mapped to a single public
IP address. The goal of PAT is to conserve IP addresses or to publish multiple hosts with service
to the internet while having only one single IP assigned on the external side of your gateway.
Most home networks use PAT. In such a scenario, the Internet Service Provider (ISP) assigns a
single IP address to the home network's router. When Computer X logs on the Internet, the router
assigns the client a port number, which is appended to the internal IP address. This, in effect,
gives Computer X a unique address. If Computer Z logs on the Internet at the same time, the
router assigns it the same local IP address with a different port number. Although both computers
are sharing the same public IP address and accessing the Internet at the same time, the router
knows exactly which computer to send specific packets to because each computer has a unique
internal address.
Port Address Translation is also called porting, port overloading, port-level multiplexed NAT and
single address NAT.
Shon Harris has the following example in her book:
The company owns and uses only one public IP address for all systems that need to communicate
outside the internal network. How in the world could all computers use the exact same IP
address? Good question. Here's an example: The NAT device has an IP address of 127.50.41.3.
When computer A needs to communicate with a system on the Internet, the NAT device
documents this computer's private address and source port number (10.10.44.3; port 43,887). The
NAT device changes the IP address in the computer's packet header to 127.50.41.3, with the
source port 40,000. When computer B also needs to communicate with a system on the Internet,
the NAT device documents the private address and source port number (10.10.44.15; port 23,398)
and changes the header information to 127.50.41.3 with source port 40,001. So when a system
responds to computer A, the packet first goes to the NAT device, which looks up the port number
40,000 and sees that it maps to computer A's real information. So the NAT device changes the
header information to address 10.10.44.3 and port 43,887 and sends it to computer A for
processing. A company can save a lot more money by using PAT, because the company needs to
buy only a few public IP addresses, which are used by all systems in the network.
As mentioned on Wikipedia:
NAT is also known as Port Address Translation: is a feature of a network device that translate
TCP or UDP communications made between host on a private network and host on a public
network. I allows a single public IP address to be used by many host on private network which is
usually a local area network LAN
NAT effectively hides all TCP/IP-level information about internal hosts from the Internet.
The following were all incorrect answer:
IP Spoofing - In computer networking, the term IP address spoofing or IP spoofing refers to the
creation of Internet Protocol (IP) packets with a forged source IP address, called spoofing, with the
purpose of concealing the identity of the sender or impersonating another computing system.
Subnetting - Subnetting is a network design strategy that segregates a larger network into smaller
components. While connected through the larger network, each subnetwork or subnet functions
with a unique IP address. All systems that are assigned to a particular subnet will share values
that are common for both the subnet and for the network as a whole.
A different approach to network construction can be thought of as subnetting in reverse. Known as
CIDR, or Classless Inter-Domain Routing, this approach also creates a series of subnetworks.
Rather than dividing an existing network into small components, CIDR takes smaller components
and connects them into a larger network. This can often be the case when a business is acquired
by a larger corporation. Instead of doing away with the network developed and used by the newly
acquired business, the corporation chooses to continue operating that network as a subsidiary or
an added component of the corporation's network. In effect, the system of the purchased entity
becomes a subnet of the parent company's network.
IP Distribution - This is a generic term which could mean distribution of content over an IP network
or distribution of IP addresses within a Company. Sometimes people will refer to this as Internet
Protocol address management (IPAM) is a means of planning, tracking, and managing the Internet
Protocol address space used in a network. Most commonly, tools such as DNS and DHCP are
used in conjunction as integral functions of the IP address management function, and true IPAM
glues these point services together so that each is aware of changes in the other (for instance
DNS knowing of the IP address taken by a client via DHCP, and updating itself accordingly).
Additional functionality, such as controlling reservations in DHCP as well as other data
aggregation and reporting capability, is also common. IPAM tools are increasingly important as
new IPv6 networks are deployed with larger address pools, different subnetting techniques, and
more complex 128-bit hexadecimal numbers which are not as easily human-readable as IPv4
addresses.
Reference(s) used for this question:
STREBE, Matthew and PERKINS, Charles, Firewalls 24seven, Sybex 2000, Chapter 1:
Understanding Firewalls.
Schneiter, Andrew (2013-04-15). Official (ISC)2 Guide to the CISSP CBK, Third Edition :
Telecommunications and Network Security, Page 350.
Harris, Shon (2012-10-25). CISSP All-in-One Exam Guide, 6th Edition (Kindle Locations 12765-
12774). Telecommunications and Network Security, Page 604-606
http://searchnetworking.techtarget.com/definition/Port-Address-Translation-PAT
http://en.wikipedia.org/wiki/IP_address_spoofing
http://www.wisegeek.com/what-is-subnetting.htm
http://en.wikipedia.org/wiki/IP_address_management
