DES works with 64 bit blocks of text using a 64 bit key (with 8 bits used for parity, so
the effective key length is 56 bits).
Some people are getting the Key Size and the Block Size mixed up. The block size is usually a
specific length. For example DES uses block size of 64 bits which results in 64 bits of encrypted
data for each block. AES uses a block size of 128 bits, the block size on AES can only be 128 as
per the published standard FIPS-197.
A DES key consists of 64 binary digits ("0"s or "1"s) of which 56 bits are randomly generated and
used directly by the algorithm. The other 8 bits, which are not used by the algorithm, may be used
for error detection. The 8 error detecting bits are set to make the parity of each 8-bit byte of the
key odd, i.e., there is an odd number of "1"s in each 8-bit byte1. Authorized users of encrypted
computer data must have the key that was used to encipher the data in order to decrypt it.
IN CONTRAST WITH AES
The input and output for the AES algorithm each consist of sequences of 128 bits (digits with
values of 0 or 1). These sequences will sometimes be referred to as blocks and the number of bits
they contain will be referred to as their length. The Cipher Key for the AES algorithm is a
sequence of 128, 192 or 256 bits. Other input, output and Cipher Key lengths are not permitted by
this standard.
The Advanced Encryption Standard (AES) specifies the Rijndael algorithm, a symmetric block
cipher that can process data blocks of 128 bits, using cipher keys with lengths of 128, 192, and
256 bits. Rijndael was designed to handle additional block sizes and key lengths, however they
are not adopted in the AES standard.
The AES algorithm may be used with the three different key lengths indicated above, and
therefore these different "flavors" may be referred to as "AES-128", "AES-192", and "AES-256".
The other answers are not correct because:
"64 bits of data input results in 56 bits of encrypted output" is incorrect because while DES does
work with 64 bit block input, it results in 64 bit blocks of encrypted output.
"128 bit key with 8 bits used for parity" is incorrect because DES does not ever use a 128 bit key.
"56 bits of data input results in 56 bits of encrypted output" is incorrect because DES always works
with 64 bit blocks of input/output, not 56 bits.
Reference(s) used for this question:
Official ISC2 Guide to the CISSP CBK, Second Edition, page: 336-343 http://csrc.nist.gov/publications/fips/fips197/fips-197.pdf http://csrc.nist.gov/publications/fips/fips46-3/fips46-3.pdf

Auditing tools are technical controls that track activity within a network on a network device or on a specific computer. Even though auditing is not an activity that will deny an entity access to a network or computer, it will track activities so a network administrator can understand the types of access that took place, identify a security breach, or warn the administrator of suspicious activity. This can be used to point out weakness of their technical controls and help administrators understand where changes need to be made to preserve the necessary security level within the environment. . - Shon Harris All-in-one CISSP Certification Guide pg 179-180

In general, IT security measures are tailored according to an organization's unique needs. While numerous factors, such as the overriding mission requirements, and guidance, are to be considered, the fundamental issue is the protection of the mission or business from IT security-related, negative impacts. Because IT security needs are not uniform, system designers and security practitioners should consider the level of trust when connecting to other external networks and internal sub-domains. Recognizing the uniqueness of each system allows a layered security strategy to be used - implementing lower assurance solutions with lower costs to protect less critical systems and higher assurance solutions only at the most critical areas.
The more complex the mechanism, the more likely it may possess exploitable flaws.
Simple mechanisms tend to have fewer exploitable flaws and require less maintenance.
Further, because configuration management issues are simplified, updating or replacing a simple mechanism becomes a less intensive process.
Security designs should consider a layered approach to address or protect against a specific threat or to reduce a vulnerability. For example, the use of a packet-filtering router in conjunction with an application gateway and an intrusion detection system combine to increase the work-factor an attacker must expend to successfully attack the system. Adding good password controls and adequate user training improves the system's security posture even more.
The need for layered protections is especially important when commercial-off-the-shelf
(COTS) products are used. Practical experience has shown that the current state-of-the-art for security quality in COTS products does not provide a high degree of protection against sophisticated attacks. It is possible to help mitigate this situation by placing several controls in series, requiring additional work by attackers to accomplish their goals.
Source: STONEBURNER, Gary & al, National Institute of Standards and Technology
(NIST), NIST Special Publication 800-27, Engineering Principles for Information
Technology Security (A Baseline for Achieving Security), June 2001 (pages 9-10).

"The following are the three most common VPN communications protocol standards:
Point-to-Point Tunneling Protocol(PPTP). PPTP works at the Data Link Layer of the OSI model. Designed for individual client to server connections, it enables only a single point-to-point connection per session. This standard is very common with asynchronous connections that use Win9x or NT clients. PPTP uses native Point-to-Point Protocol (PPP) authentication and encryption services.
Layer 2 Tunneling Protocol (L2TP). L2TP is a combination of PPTP and the earlier Layer 2 Forwarding (L2F) Protocol that works at the Data Link Layer like PPTP. It has become an accepted tunneling standard for VPN's. In fact, dial-up VPNs use this standard quite frequently. Like PPTP, this standard was designed for single point-to-point client to server connections. Not that multiple protocols can be encapsulated within the L2TP tunnel, but do not use encryption like PPTP. Also, L2TP supports TACACS+ and RADIUS, but PPTP does not.
IPSEC. IPSec operates at the Network Layer and it enables multiple and simultaneous tunnels, unlike the single connection of the previous standards. IPSec has the functionality to encrypt and authenticate IP data. It is built into the new Ipv6 standard, and is used as an add-on to the current Ipv4. While PPTP and L2TP are aimed more at dial-up VPNs, IPSec focuses more on network-tonetwork connectivity." Pg. 123-125 Krutz: The CISSP Prep Guide: Gold Edition.