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Chapter Overview

Subnet

What is a subnet

When you break a network into a few smaller networks, you have created several subnets

Like IP address where we have network addresses and host IDs, we now have subnet addresses and host IDs

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What Is a Subnet Mask?

A subnet mask is a 32-bit binary number that indicates which bits of an IP address identify the network and which bits identify the host.

The 1 bits are the network identifier bits and the 0 bits are the host identifier bits.

A subnet mask is typically expressed in dotted decimal notation.

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Subnet Masks for IP Address Classes

Class Subnet Mask

A 255.0.0.0

B 255.255.0.0

C 255.255.255.0

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Creating Subnets

Borrow bits from the host identifier and use them as a subnet identifier.

Convert the binary values to decimals.

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Subnetting a Class B Address

Classless Inter-Domain Routing (CIDR)

(CIDR) is an Internet Protocol (IP) address allocation and route aggregation methodology used within the Internet addressing architecture that replaced the IPv4 classful network organization of the IP address space.

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CIDR notation

In the CIDR notation, the IP address is expressed according to the standards of IPv4 or IPv6, followed by the separator character which is a slash (/), followed by the prefix size as a decimal number.

the address specification 192.168.100.1/24 represents the given IPv4 address and its associated routing prefix 192.168.100.0 or, equivalently, its subnet mask, 255.255.255.0.

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CIDR notation (2)

You cannot have 192.168.100.1/31 because the hosts need to have at least two bits

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The five key questions of a subnet mask

How many subnets does a subnet mask produce?

Number of valid hosts per subnet What are the valid subnets What are the broadcast address of each

subnet What are the valid hosts in a subnebt

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How many subnets

2x where x is the number of 1s in the subnet mask after the 1s for the traditional class (8 for A, 16 for B, and 24 for C)

For example, the subnet mask 255.255.255.192 would have 22=4 subnet for a class C network.

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Number of valid hosts per subnet

2y -2, where y is the number of zeros in a subnet mask, is the number of hosts

The -2 are for all 0s and all 1s. For 255.255.255.192, the number of

hosts are 26 -2 = 62

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What are the valid subnets

For 255.255.255.192 on a class C network they are 0, (00000000 to 00111111) 64, (01000000 to 01111111) 128, (10000000 to 10111111) and 192 , (11000000 to 11111111)

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What are the broadcast address

For 255.255.255.192 they are 0 00111111 64 01111111 128 10111111 192 11111111

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What are the valid hosts

For 255.255.255.192 they are 0, (00000001 to 00111110) 64, (01000001 to 01111110) 128, (10000001 to 10111110) and 192 , (11000001 to 11111110)

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Example #1C (Page 219) On mask of 255.255.255.128/25

# of subnet 2 Hosts in each subnet 126 The two subnets are 0 and 128, which broadcast

address be 127 and 255 respectively Valid host addresses

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Assigning IP addresses

Note, this 192.168.0.0 ~192.168.255.255 are class C private IP addresses

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Troubleshooting

1. Ping 127.0.0.1 (or ::1) If cannot connect, host problems

NIC, TCP/IP, subnet mask, etc Ping IP address of the host

If works, NIC is working Ping the default gateway

If works, connected to LAN If not, most likely physical layer issues

Ping server If working, the server is working, so is IP If not, server could be down, or others

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What’s wrong here (pp. 237)

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It is a broadcast address

Tools

Ping (Packet Internet groper) Traceroute/tracert Arp –a Ipconfig

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Network Address Translation (NAT)

Access the Internet with private IP address

Change ISP without renumbering computer’s IP addresses

Merge two networks with some duplicated IP addresses

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NAT where?

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Good and bad

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Three types of translations

Static – allow one to one mapping of local and global address, use to hide internal addresses

Dynamic – doing the same as Static, just need a pool of real IP addresses

Overloading – most interesting, used by home Internet routers – using ports

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Example of overloading

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