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Thread: In preparation to write my CompTIA A+ Certification....

  1. #31
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    Re: In preparation to write my CompTIA A+ Certification....

    Quote Originally Posted by Drone4four View Post
    I’m not really sure what specifically mips was referring to with respect to his comment about how useless certs are based on what he read in this thread.

    I have encountered three further points of clarification in the networking material.

    #1
    The 1st, 2nd, 3rd octets of an IP address is the network ID. The 4th octet is the host ID.
    But devices are usually assigned two of these addresses: First, an IP address and second, a subnet mask. A device with a DHCP client installed on a host could receive, as an example, both of the following from a DHCP server:
    Code:
    192.168.1.41 (IP)
    255.255.255.0 (subnet mask)
    The network ID for the IP is 192.168.1 and the host ID is 41. The network ID for the subnet mask is 255.255.255 and the host Id is 0. Is this accurate?
    Not really. The subnet mask is the key that deciphers which part of the IP is network address and which part is host address. So I can have an address that's 10.10.20.1 with a mask of 255.255.0.0, and there the network id is 10.10.0.0 and the host id is 20.1. If I change the mask to 255.255.255.0, the host id is now just .1 and the network is 10.10.20.0.
    It doesn't have to break on the octet either; for example, the network I'm on now has a subnet mask of 255.255.252.0.

    Another way you'll see this notated is called CIDR notation, which looks like 10.10.20.0/24. The "24" is equivalet to 255.255.255.0, because it means the first 24 bits of the IP is the network. My subnet mask of 255.255.252.0 would be /22 in CIDR notation.

    The principle reason why devices on networks are usually assigned IPs and masks is because, so the internet’s architects reasoned, it would be mathematically impossible for every device to have a unique address because there are more devices than are available IP addresses:

    Code:
    (16,000,000×127)+(65,000×16,000)+(254×2,000,000) = 3,580,000,000
    The multiplication in the first set of brackets represent Class A, the second set represents Class B and the third, Class C. Class D is multicast and E is reserved. CIDR and IETF in 1993 introduced IPv4, among other things, including subnet masking for the purposes of providing billions of additional address combinations to accommodate for the more than 3.5 billion devices/servers/gateways demanded by consumers, businesses and governments.

    Is all that accurate?
    The way we currently accomodate billions of connected devices is by putting the vast majority of them on a private IP range behind a NAT. Getting an actual public IP usually involves some expense, and we're running out of them. This is why you'll see people pushing for ipv6, which adds a lot more addresses and does away with the need to NAT and do private addressing.
    #2
    After installing an all-in-one printer/fax/copier, does it use an APIPA address until the network admin assign it a more permanent address using the gateway router’s firewall firmware? And please correct me if I am wrong, but most all-in-one printer/fax/copiers usually don’t have a DHCP client installed to use to exchange a lease agreement. This means that a network admin has to assign it a static IP. Is that accurate?
    Actually most network-connected devices these days does have a dhcp client built in; I'd say most printers I've seen for at least the last decade can do DHCP. It's probably a good idea to assign a static IP so you know where to send your print jobs, though.

    #3
    Wikipedia says that IANA is responsible for the global coordination of the DNS Root, IP addressing, and other Internet protocol resources. *Is IANA the body (which would be many years ago now) that decided that FTP would use port 21, http use port 80, and irc port 194? *Is it also possible that Valve software, for their first multiplayer game they wrote (Half-Life) back in 1997, was told by IANA to use port 27015?
    Only if it is an officially registered port number. See https://en.wikipedia.org/wiki/List_o...P_port_numbers

    Some of you might be thinking, ‘why is this person asking CCNA/Network+ questions when he is preparing to write his A+ exams’? There are 3 chapters in both of my A+ texts devoted to networking. These chapters in particular I struggled with the most, compared to all the other chapters on PC repair and such. I am working on my weakest link by trying to understand these basic network concepts....
    If you want to go into IT (I don't know why you'd want an A+ if you didn't), it's best to get these concepts straight whether or not they're on the exam. Every computer is networked these days, there isn't much point in a tech who doesn't understand networking.

  2. #32
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    Re: In preparation to write my CompTIA A+ Certification....

    CIDR really threw me for a loop before I actually understood why it was used for (custom subnetting ftw!).
    Last edited by CharlesA; June 14th, 2013 at 11:35 PM. Reason: custom, not customer.. derp
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  3. #33
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    Re: In preparation to write my CompTIA A+ Certification....

    Quote Originally Posted by CharlesA View Post
    CIDR really threw me for a loop before I actually understood why it was used for (customer subnetting ftw!).
    We would have run out of IP addresses way before NAT came along if it was not for CIDR.

  4. #34
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    Re: In preparation to write my CompTIA A+ Certification....

    Aye, classful subnetting was a waste of addresses.
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  5. #35
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    Re: In preparation to write my CompTIA A+ Certification....

    @lykwydchykyn: Thanks for the clarifications on IANA and on printers. Give me a few days to reply to what you said about NAT, subnetting, and CIDR notation.
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  6. #36
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    Re: In preparation to write my CompTIA A+ Certification....

    Wikipedia uses this example:

    IP address: 192.168.5.130
    Subnet mask: 255.255.255.0
    Network prefix: 192.168.5.0
    Host part: 0.0.0.130
    Wikipedia uses the expression, ‘host part’. Is this the same as, ‘host ID’? And is ‘network prefix’ the same as ‘net id’? I’ll assume it is until someone corrects me. Here is the example I initially used, but reorganized into the categories described in lykwydchykyn’s post and on Wikipedia:

    IP address: 192.168.1.41
    Subnet mask: 255.255.255.0
    Network ID: 192.168.1.0
    Host ID: 0.0.0.41
    Likewise, in both examples, the CIDR notation would be the IP address, with /24 to indicate the subnet mask in shorthand.

    After receive feedback from you folks a few days ago I found a guide on CIDR notation in the knowledge base section on whatismyipaddress.com. After reading it over a few times, I did my best to practice by making up some address combinations and trying to reformat the subnet masks into shorthand CIDR notation. Here it is below:

    IP: 192.168.1.41, subnet mask: 3.128.46.1,my CIDR notation: 192.168.1.41 /0
    IP: 192.168.1.41, subnet mask: 129.190.6.12, my CIDR notation: 192.168.1.41 /1
    IP: 192.168.1.41, subnet mask: 219.0.255.3.9, my CIDR notation: 192.168.1.41 /3
    IP: 192.168.1.41, subnet mask: 0.128.46.1, my CIDR notation: 192.168.1.41 /0
    IP: 192.168.1.41, subnet mask: 0.0.46.1, my CIDR notation: 192.168.1.41 /0
    IP: 192.168.1.41, subnet mask: 0.10.46.1, my CIDR notation: 192.168.1.41 /0
    IP: 192.168.1.41, subnet mask: 255.128.46.1, my CIDR notation: 192.168.1.41 /9
    IP: 192.168.1.41, subnet mask: 255.255.46.1, my CIDR notation: 192.168.1.41 /16
    IP: 192.168.1.41, subnet mask: 255.255.253.1, my CIDR notation: 192.168.1.41 /22
    IP: 192.168.1.41, subnet mask: 255.255.124.1, my CIDR notation: 192.168.1.41 /16

    After my study session practicing with this exercise I came to the conclusion that it was all wrong. But here I am a few days later and I can’t see what’s wrong with it. Can someone please enlighten me?

    The purpose of dividing networks up into subnets below a NAT gateway router (along with addresses with CIDR notations) is to allow for devices to access the internet while managing the scarcity of IPv4 addresses. With respect to NATs, I found a badass entry on the Azureus wiki. I don’t understand it completely. After reading that article I began to draw a similarity between NATs and proxies. They sorta seemed the same, but I have since discovered some major differences. One difference is that NAT is transparent, meaning that the destination from outside the network can see the source. Whereas the purpose of a proxy server is to conceal the identity from which information is being requested from. The external address being navigated to is convinced that the proxy IS the authentic source of the information request. NATs are a little more honest. With NATs, it’s possible that the external address being navigated can determine the true source. Proxies hide the source address. NATs don’t.If all that is true, then why does SAM’s Teach Yourself TCP/IP authored by Joe Casad claim the reverse is true? Joe Casad says, “A NAT device obscures all details of the local network and, in fact, hides the existence of the local network.” Here is a few paragraphs so you have the complete context:

    Some experts began to notice that, if a DHCP server is providing the client with an IP address, there is no real reason why this address has to be an official, unique “legal” Internet address. As long as the router itself has an Internet-ready address, it can act as a proxy for clients on the network—receiving requests from clients and translating the requests to and from the Internet address space. Many router/DHCP devices today also perform a service known as Network Address Translation (NAT).

    ...

    A NAT device improves security because it can prevent an outside attacker from finding out about the local network. To the outside world, the NAT device looks like a single host connected to the Internet. Even if an attacker knew the address of a computer on the local network, the attacker would not be able to open a connection with the local network because the local addressing scheme is not contiguous with the Internet address space.
    Do all gateway routers act as NATs?
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  7. #37
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    Re: In preparation to write my CompTIA A+ Certification....

    Quote Originally Posted by Drone4four View Post
    IP: 192.168.1.41, subnet mask: 3.128.46.1,my CIDR notation: 192.168.1.41 /0
    IP: 192.168.1.41, subnet mask: 129.190.6.12, my CIDR notation: 192.168.1.41 /1
    IP: 192.168.1.41, subnet mask: 219.0.255.3.9, my CIDR notation: 192.168.1.41 /3
    IP: 192.168.1.41, subnet mask: 0.128.46.1, my CIDR notation: 192.168.1.41 /0
    IP: 192.168.1.41, subnet mask: 0.0.46.1, my CIDR notation: 192.168.1.41 /0
    IP: 192.168.1.41, subnet mask: 0.10.46.1, my CIDR notation: 192.168.1.41 /0
    IP: 192.168.1.41, subnet mask: 255.128.46.1, my CIDR notation: 192.168.1.41 /9
    IP: 192.168.1.41, subnet mask: 255.255.46.1, my CIDR notation: 192.168.1.41 /16
    IP: 192.168.1.41, subnet mask: 255.255.253.1, my CIDR notation: 192.168.1.41 /22
    IP: 192.168.1.41, subnet mask: 255.255.124.1, my CIDR notation: 192.168.1.41 /16

    After my study session practicing with this exercise I came to the conclusion that it was all wrong. But here I am a few days later and I can’t see what’s wrong with it. Can someone please enlighten me?
    No idea as we have no idea as to how you arrived at those results. Maybe go back to the basics and start over.

  8. #38
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    Re: In preparation to write my CompTIA A+ Certification....

    Quote Originally Posted by Drone4four View Post
    Wikipedia uses this example:



    Wikipedia uses the expression, ‘host part’. Is this the same as, ‘host ID’? And is ‘network prefix’ the same as ‘net id’? I’ll assume it is until someone corrects me. Here is the example I initially used, but reorganized into the categories described in lykwydchykyn’s post and on Wikipedia:
    Same thing. As long as you can tell which part of the IP is the host part and which is the network part, you are ok.



    Likewise, in both examples, the CIDR notation would be the IP address, with /24 to indicate the subnet mask in shorthand.

    After receive feedback from you folks a few days ago I found a guide on CIDR notation in the knowledge base section on whatismyipaddress.com. After reading it over a few times, I did my best to practice by making up some address combinations and trying to reformat the subnet masks into shorthand CIDR notation. Here it is below:

    IP: 192.168.1.41, subnet mask: 3.128.46.1,my CIDR notation: 192.168.1.41 /0
    IP: 192.168.1.41, subnet mask: 129.190.6.12, my CIDR notation: 192.168.1.41 /1
    IP: 192.168.1.41, subnet mask: 219.0.255.3.9, my CIDR notation: 192.168.1.41 /3
    IP: 192.168.1.41, subnet mask: 0.128.46.1, my CIDR notation: 192.168.1.41 /0
    IP: 192.168.1.41, subnet mask: 0.0.46.1, my CIDR notation: 192.168.1.41 /0
    IP: 192.168.1.41, subnet mask: 0.10.46.1, my CIDR notation: 192.168.1.41 /0
    IP: 192.168.1.41, subnet mask: 255.128.46.1, my CIDR notation: 192.168.1.41 /9
    IP: 192.168.1.41, subnet mask: 255.255.46.1, my CIDR notation: 192.168.1.41 /16
    IP: 192.168.1.41, subnet mask: 255.255.253.1, my CIDR notation: 192.168.1.41 /22
    IP: 192.168.1.41, subnet mask: 255.255.124.1, my CIDR notation: 192.168.1.41 /16
    That doesn't look right.

    192.168.1.0/24 = 255.255.255.0
    192.168.1.0/25 = 255.255.255.128

    Have a read here:
    http://www.tcpipguide.com/free/t_IPC...ingExample.htm

    It also helps to break the notation down into it's binary form. That's how I learned CIDR.

    Do all gateway routers act as NATs?
    I don't know for sure, but I doubt it. Depends on the device.
    Last edited by CharlesA; June 17th, 2013 at 07:59 PM.
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  9. #39
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    Re: In preparation to write my CompTIA A+ Certification....

    You are never going to have a subnet mask like 0.0.46.1. The point of a subnet mask is to tell you how many bits starting on the left define the network address (which is why we can just abbreviate it to a single number between 0 and 32). So in binary it's always going to look like a certain number of 1's followed by a certain number of 0's. So a netmask octet won't just be any arbitrary number from 0-255.

    NAT and proxying operate on different layers; NAT on the logical addressing layer, proxying on the application layer.

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