IPv4 Addressing - 1

(Updated: )
IT

Recall Layer 3: Network Layer

  • Provides connectivity between end hosts on different networks (outside of the LAN)
  • Provides logical addressing (IP addresses)
  • Provides path selection between source and destination.
  • Routers operate at Layer 3

Routing

  • Recall switches simply expand networks. Therefore, PCs within the same LAN have the same IP addresses within the same network.

What does the IP address indicate?

  • The three group 192.168.X indicates network itself.
  • Y in 192.168.X.Y represents the PCs/clients/servers.
  • The /24 represents the network and which part represents the end-host.
    • /24 says the first 3 group of the numbers represents the network.

The router in the picture above requires an IP address.

  • G0/0 interface is given 192.168.1.254
  • G0/1 interface is given 192.168.2.254
  • The broadcast signal is forwarded to 192.168.1.2 and 192.168.1.254.
    • Broadcast signals are limited within the local network.

IPv4 Header (Let’s focus on Source and Dest IP address atm)

  • IP addresses are 32 bits (4 bytes) in length.

192.168.1.254: A closer look

  • Each group of numbers represents 8 bits.
    • 192 = 8 bits => 11000000
    • 168 = 8 bits => 10101000
    • 1 = 8 bits => 00000001
    • 254 = 8 bits = 11111110
  • Instead of binary, we use dotted decimal.

Recall Decimal, Hexadecimal, and Binary Notation


Decimal to Binary…

The range of possible numbers of binary can range from?

0 - 255

IPv4 address is a series of 32 bits

So what is the /24?

  • The /24 indicates the first 24 bits represents the network portion, and the remaining 8 represents the host.

/16 indicates the first half.

/8

IPv4 Address Classes

Class First octet First octet numeric range
A 0xxxxxxx 0-127 64+32+16+8+4+2+1 = 127 Really 0 - 126, not 127.
B 10xxxxxx 128 - 191 128 + 32 + 16 + 8 + 4 + 2 + 1 = 191
C 110xxxxx 192-223
D 1110xxxx 224-239 Reserved for multicast (different from unicast and broadcast)
E 1111xxxx 240-255 Reserved for experimental use

Why is class A range in practice only 0 - 126? Loopback Addresses

  • The 127 octet range is reserved for loopback addresses. What does that mean?
    • The first octet is always 127.
      • Address range 127.0.0.0 - 127.255.255.255
    • Used to test the ‘network stack’ (think OSI, TCP/IP model) on the local device.
      • If a device sends any network traffic in this range, it’s simply sent back up the network stack (think of when you receive a packet and it’s being de-encapsulated).
    • This is demonstrated by the RTT when pinged.

Implications of Class A, B, C.

Class First octet First octet numeric range Prefix Length
A 0xxxxxxx 0-127 * really 0 - 126 /8
B 10xxxxxx 128 - 191 /16
C 110xxxxx 192-223 /24

  • Class A: Fewer potential networks, there can be many hosts on each network

  • Class C: there are many possible networks, but there can only be a few hosts.

  • Indicated by the chart above, class C networks can only have 256 possible hosts.
    • However, the first bit is reserved for the network. AND the last address of the network is the broadcast network (the layer 3 address when you want to send traffic to all host
    • So really, the host count is 2 LESS. 256 - 2 = 254 in class C

Netmask: “A Newer and Easier Way of Writing the Prefix Length?”

In juniper networks, we use the slash notation.

  • Class A: /8
  • Class B: /16
  • Class C: /24

Cisco networks use an older method: dotted decimal netmask

  • Class A: 255.0.0.0 (11111111 00000000 00000000 00000000)
  • Class B: 255.255.0.0 (11111111 11111111 00000000 00000000)
  • Class C: 255.255.255.0 (11111111 11111111 11111111 00000000)

The network address CANNOT be assigned to the host.

  • If the host portion of the address is all 0’s, then it is a network address.

192.168.1.0/24 = network address
192.168.2.0/24 = network address

The first usable host address is 192.168.1.1/24

The last address in the network is the broadcast address.

  • If the host portion of the address is all 1’s, then it is the broadcast address.

192.168.1.255/24 (X.X.X.1111111)

The last usable host address is 192.168.1.254/24

In short, we talk about

  • Dotted decimal and binary
  • Network portion / host portion of IPv4
  • IPv4 address classes
  • Prefix lengths / netmasks (Cisco)
  • Network address / broadcast address

Question 0

If we sent a ping to 192.168.1.255, what would be the destination MAC address?

  • Given that 255 is the broadcast address, the destination MAC address should be FFFF.FFFF.FFFF