IP Addressing and Subnetting

• IPv4 = 32 bits as four octets (0-255), dotted decimal
• Each octet is 8 bits, so 4 x 8 = 32
• Mask says where the network part ends and the host part begins

ip = '192.168.10.37'
print('.'.join(f'{int(o):08b}' for o in ip.split('.')))

each octet as 8 bits

• 127.x.x.x is loopback, which is why A stops at 126
• Waste: a /8 gives ~16M hosts even if you need 300
• That waste is why CIDR replaced classful addressing

• Mask = prefix bits set to 1, rest 0
• Network = IP AND mask (host bits all 0)
• Broadcast = host bits all 1
• Usable hosts = 2^h - 2, h = host bits

• Block size = 256 − mask octet, in the interesting octet
• /26: 256 − 192 = 64, so blocks .0 .64 .128 .192

• /31: point-to-point, 2 usable addresses (RFC 3021)
• /32: a single host route

• Interview Point
• Hosts in a /26? 62 = 2^6 − 2
• Network of 192.168.1.130/26? 192.168.1.128, broadcast .191
• Why does /31 work? Point-to-point needs no broadcast (RFC 3021)

• Step 1: smallest h with 2^h - 2 >= hosts
• 50 hosts: h=5 gives 30, h=6 gives 62
• Step 2: prefix = 32 - h, so /26
• Borrow n bits for 2^n subnets: /24 borrow 2 = four /26s
• Step 3: network = previous broadcast + 1

• 192.168.0.0/24 + 192.168.1.0/24 = 192.168.0.0/23
• Only if contiguous and aligned

• RFC 1918 ranges are not routable on the public internet
• 172.32.x.x is public, not private

• Breaks end-to-end addressing
• Complicates inbound connections

• 128 bits = eight groups of 4 hex digits, split by :
• Drop leading zeros in each group
• Replace one run of zero groups with ::
• :: is allowed only once

2001:0db8:0000:0000:0000:ff00:0042:8329
2001:db8::ff00:42:8329

same address, compressed

• IPv4's ~4.3 billion addresses are exhausted
• IPv6 gives 2^128 addresses
• Enough public addresses, so no NAT needed

• Typical subnet is a /64
• ip addr or ipconfig shows your address
• ping -6 tests IPv6
• No broadcast: it uses multicast

• Network = IP AND mask
• Broadcast = all host bits 1
• Hosts = 2^h - 2, h = host bits
• Block size = 256 - mask octet

• NAT/PAT rewrites private to public, PAT uses ports
• IPv6 = 128-bit hex, :: compresses one zero run
• IPv6 has no broadcast and needs no NAT