This is a valid Class A reserved address. For Class A, the reserved addresses are 10.0.0.0 - 10.255.255.255.
The following answers are incorrect:
11.0.42.5 Is incorrect because it is not a Class A reserved address.
12.0.42.5 Is incorrect because it is not a Class A reserved address.
13.0.42.5 Is incorrect because it is not a Class A reserved address.
The private IP address ranges are defined within RFC 1918:
RFC 1918 private ip address range
References:
3Com http://www.3com.com/other/pdfs/infra/corpinfo/en_US/501302.pdf
AIOv3 Telecommunications and Networking Security (page 438)

The Computer Security Policy Model Orange Book is based is the Bell-
LaPadula Model. Orange Book Glossary.
The Data Encryption Standard (DES) is a cryptographic algorithm. National Information
Security Glossary.
TEMPEST is related to limiting the electromagnetic emanations from electronic equipment.
Reference: U.S. Department of Defense, Trusted Computer System Evaluation Criteria
(Orange Book), DOD 520028-STD. December 1985 (also available here).

The principle of accountability has been described in many references; it is a principle by which specific action can be traced back to an individual. As mentioned by Idrach, any significant action should be traceable to a specific user. The definition of "Significant" is entirely dependant on your business circumstances and risk management model. It was also mentioned by Rino that tracing the actions of a specific user is fine but we must also be able to ascertain that this specific user was responsible for the uninitiated action.

The server also checks the authenticator and, if that timestamp is valid, it provides the requested service to the client.
Even if the user principal is present in a ticket and only the application server can extract and possibly manage such information (since the ticket is encrypted with the secret key of the service), this is not enough to guarantee the authenticity of the client.
An impostor could capture (remember the hypothesis of an open and insecure network) the ticket when it is sent by a legitimate client to the application server, and at an opportune time, send it to illegitimately obtain the service.
On the other hand, including the IP addresses of the machine from where it is possible to use it is not very useful: it is known that in an open and insecure network addresses are easily falsified. To solve the problem, one has to exploit the fact that the client and server, at least during a session have the session key in common that only they know (also the
KDC knows it since it generated it, but it is trusted by definition!!!).
Thus the following strategy is applied: along with the request containing the ticket, the client adds another packet (the authenticator) where the user principal and time stamp (its at that time) are included and encrypts it with the session key; the server which must offer the service, upon receiving this request, unpacks the first ticket, extracts the session key and, if the user is actually who he/she says, the server is able to unencrypt the authenticator extracting the timestamp.
If the latter differs from the server time by less than 2 minutes (but the tolerance can be configured) then the authentication is successful. This underlines the criticality of synchronization between machines belonging to the same realm.
The Replay Attack
A replay attack occurs when an intruder steals the packet and presents it to the service as if the intruder were the user. The user's credentials are there -- everything needed to access a resource. This is mitigated by the features of the "Authenticator," which is illustrated in the picture below.
The Authenticator is created for the AS_REQ or the TGS_REQ and sends additional data, such as an encrypted IP list, the client's timestamp and the ticket lifetime. If a packet is replayed, the timestamp is checked. If the timestamp is earlier or the same as a previous authenticator, the packet is rejected because it's a replay. In addition, the time stamp in the
Authenticator is compared to the server time. It must be within five minutes (by default in
Windows).
Kerberos Authenticator to prevent replay attacks
The Authenticator mitigates the Possibility of a replay attack.
If the time skew is greater than five minutes the packet is rejected. This limits the number of possible replay attacks. While it is technically possible to steal the packet and present it to the server before the valid packet gets there, it is very difficult to do.
It's fairly well known that all computers in a Windows domain must have system times within five minutes of each other. This is due to the Kerberos requirement.
Reference(s) used for this question:
Redmond Magazine
and
http://kerberos.org/software/tutorial.html
and
KRUTZ, Ronald L. & VINES, Russel D., The CISSP Prep Guide: Mastering the Ten
Domains of Computer Security, 2001, John Wiley & Sons, Page 42