A proposito dell'autore

Trentatre anni, perito informatico, professionista da dieci nel mondo ICT, possiede varie certificazioni - nel mondo Cisco è CCNA da Dicembre. Lavora nel mondo Unix, Cisco ed Oracle per una delle maggiori telco a Düsseldorf (Germania) come deployment leader e responsabile della sicurezza. E' generalmente entusiasta di condividere le sue conoscenze e la sua visione del mondo con altri tecnici.

  • * Classes of IP addresses
  • Class

    Possible networks

    Default mask

    Hosts

    Possible networks

    A

    1.0.0.0-126.0.0.0

    255.0.0.0

    224 – 2 = 16777214

    125 (10. private/127. loopback)

    B

    128.X.0.0-191.x.0.0

    255.255.0.0

    216 – 2 = 65534

    16K (172.16. private)

    C

    192.x.x.0-223.x.x.0

    255.255.255.0

    28 – 2 = 254

    2 millions (192.168.x private)

    D

    224.x.x.0-239.x.x.0

    255.255.255.240

    E

    240.x.x.0-255.x.x.0

    255.255.255.240

    Class

    Network vs Host

    High order bits

    CIDR notation

    First octet in decimal

    A

    N

    H

    H

    H

    0

    /8

    1-126

    B

    N

    N

    H

    H

    10

    /16

    128-191

    C

    N

    N

    N

    H

    110

    /24

    192-223

    D

    for multicast groups

    1110

    /28

    224-239

    E

    for research

    1111

    /28

    240-255

  • * Private IP addresses ß not routed to the Internet backbone
  • IP address class

    RFC 1918 Internal Address Range

    Class A

    10.0.0.0 to 10.255.255.255

    Class B

    172.16.0.0 to 172.31.255.255

    Class C

    192.168.0.0 to 192.168.255.255

    Subnetting

    Decimal notation of first octet

    No. of Subnets

    Class A hosts per subnet

    Class B hosts per subnet

    Class C hosts per subnet

    .192

    2

    4,194,302

    16,382

    62

    .224

    6

    2,097,150

    8,190

    30

    .240

    14

    1,048,574

    4,096

    14

    .248

    30

    524,286

    2,046

    6

    .252

    62

    262,142

    1,022

    2

    .254

    126

    131,070

    510

    .255

    254

    65,534

    254

    When we subnet, we borrow bits from the host part to create sub-networks.

    Example:  We have a class C class of addresses, and we want to divide it in at least 4 subnets.

    First rule of subnetting: we must borrow at least two bits, but as well at least two bits must remain.

    255.255.255.0 -> 11111111.11111111.11111111.|00000000

    | = imaginary line that divides network portion from host portion of the address

    Since we wanted 4 subnets, we need at least 3 bits (1 + 2 + 4 = 8). The new subnet will look like the following:

    11111111.11111111.11111111.111|00000 -> 255.255.255.224 (128+64+32=224) or /27

    Second rule of subnetting: number of usable subnets is 2 at the power of borrowed bits, minus 2.

    23 = 8 – 2 = 6 number of usable subnets

    Third rule of subnetting: number of usable hosts in every subnet is 2 at the power of remaining bits, minus 2.

    25 = 32 – 2 = 30 number of usable hosts for every subnet.

    Finding the subnets – fourth rule of subnetting: decimal value of the last bit borrowed = increment of the network address in the subnets table.

    We can now fill the subnets table.

    Subnet number

    Network address

    Hosts range

    Broadcast address

    Usable?

    0

    .0

    .1 – .30

    .31

    N

    1

    .32

    .33 – .62

    .63

    Y

    2

    .64

    .65 – .94

    .95

    Y

    3

    .96

    .97 – .126

    .127

    Y

    4

    .128

    .127 – .158

    .159

    Y

    5

    .160

    .161 – .190

    .191

    Y

    6

    .192

    .193 – .222

    .223

    Y

    7

    .224

    .225 – .254

    .255

    N

    First and last subnets are not usable.

    Cheat sheet – to create class B and C subnets quickly

    Bit values

    128

    64

    32

    16

    8

    4

    2

    1

    Masks

    128

    192

    224

    240

    248

    252

    254

    255

    CIDR class B

    /17

    /18

    /19

    /20

    /21

    /22

    /23

    /24

    CIDR class C

    /25

    /26

    /27

    /28

    /29

    /30

    /31

    /32

    IP’s subnet zero

    2

    4

    8

    16

    32

    64

    128

    256

    IP’s subnet non-zero

    0

    2

    6

    14

    30

    62

    126

    254

    CIDR class B

    /25

    /26

    /27

    /28

    /29

    /30

    /31

    /32

    IP’s subnet zero

    512

    1024

    2048

    4096

    8192

    16384

    32768

    65536

    IP’s subnet non-zero

    510

    1022

    2046

    4094

    8190

    16382

    32766

    65534


    To reproduce before the exam!!!

    Network calculation cheat sheet

    Increments

    128

    64

    32

    16

    8

    4

    2

    1

    128

    64

    32

    16

    8

    4

    2

    1

    CIDR

    /17

    /18

    /19

    /20

    /21

    /22

    /23

    /24

    /25

    /26

    /27

    /28

    /29

    /30

    /31

    /32

    Masks

    128

    192

    224

    240

    248

    252

    254

    255

    128

    192

    224

    240

    248

    252

    254

    255

    B subnets

    2

    4

    8

    16

    32

    64

    128

    256

    512

    1024

    2048

    4096

    8192

    16384

    32768

    65536

    C subnets

    2

    4

    8

    16

    32

    64

    128

    256

    Hosts

    32768

    16384

    8192

    4096

    2048

    1024

    512

    256

    128

    64

    32

    16

    8

    4

    2

    1

    Remove 2 from subnets and hosts when zero based is not allowed (always not allowed, unless explicitly stated otherwise).

    Classes

    A

    0

    1 – 126

    B

    10

    128 – 191

    C

    110

    192 – 223

    D

    1110

    224 – 239

    E

    11110

    240 – 255

    Private addresses

    10.x.x.x

    172.16-31.x.x

    192.168.x.x

    HEX conversion table

    Also useful to calculate /28 and /27 subnets

    1

    16

    1

    0001

    2

    32

    2

    0010

    3

    48

    3

    0011

    4

    64

    4

    0100

    5

    80

    5

    0101

    6

    96

    6

    0110

    7

    112

    7

    0111

    8

    128

    8

    1000

    9

    144

    9

    1001

    A

    160

    10

    1010

    B

    176

    11

    1011

    C

    192

    12

    1100

    D

    208

    13

    1101

    E

    224

    14

    1110

    F

    240

    15

    1111

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