Question : Problem: Roll-over cable(Urgent)!!!!

Dear Sirs,

Could you please help me to find the below questions answer?

1 ) What is this roll-over cable cable ? Can we Connect PC's com port to Router's console port through roll-over cable or Connect PC's ethernet port to Router's ethernet port through rollover cable?

2) What is the best way to find out the total host number for the given network alone? Any formula for this ?  

Question:- 1)  A company has been assigned the following network number.
172.12.0.0.
Please assign a subnet number to the company keeping in mind that there should be at least 458 hosts per network and keeping number of subnets maximum.
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Since I am running short of money to undergone any training for CCNA exam I hereby request you to kindly give some tips and suggestions in this regard. I am planning to appear for this exam by the beginning of September 1st week.

Hope you will understand and do the needful… Pl answer me the following ..
 Q1.) Which of the following belongs to the ISDN T2 function group? (Choose two)
A.) A standard PC
B.) An ISDN phone
C.) An ISDN terminal adapter
D.) A router's serial interface
E.) A router's ISDN BRI "U" interface
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Q8.) Which of the following will best define a router's function? (Choose three)
A.)Packet switching
B.) Collision prevention on LAN
C.) Packet filtering
D.) Broadcast domain enlagrement
E.) Broadcast forwarding
F.) Inter-network Communication
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Q2. A network admin was consoled into catalyst 2924 switch and noticed prompt "Switch". When any command is entered the responce of "unknown command" is returned. After entering boot command the screen displayes "error loading image". What is the most likely cause of error message. (Choose one)
A.) Startup-config file is missing
B.) IOS has been deleted or can not be loaded
C.) POST was unsuccessful
D.) RAM is corrupt
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Waiting for your reply at the earliest...

Thanks in advance
ssshibu

Answer : Problem: Roll-over cable(Urgent)!!!!

1) Rollover cable is to connect PC Com port with router Console0 poert
2) the method i use:
    for hosts you will need find the closer number which is a power of 2, for example :
    458 hosts per subnet (i choose 512 as closer number, it's 2^9) so we need 9 bits for host portion.
    the network 172.12.0.0 is class B, so it has /16 subnet mask and 16 bits for host, so we can borrow 7 bits for subbnetting.
   we will have 172.12.0.0/23 (255.255.252.0 netmask)
     number of subnets  = 2^7 - 2 = 126
                      hosts = 2^9 - 2 = 510
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