Lec 4

Centralising DHCP Servers

Configure DHCP relay agents in each building to forward DHCP packets to the DHCP servers in the ITS building.

Subnet Mask

When borrowing 3 bits, the new subnet mask is /27, allowing for 30 usable host addresses.

DHCP Server Authorisation

DHCP servers must be authorised in Active Directory to lease IP addresses; failure to do so prevents functionality.

Finding a DHCP Server

A client PC that is set to obtain an IP address automatically will send out a DHCPDISCOVER broadcast on the network to find available DHCP servers.

Supernetting

The pair 192.168.2.64/28 & 192.168.2.80/28 cannot be supernetted because they are not contiguous.

Variable Length Subnet Masks (VLSM)

Allows for subnets of different sizes within the same network for efficient IP address usage.

Creating Subnets

To create 14 subnets, 4 bits must be borrowed, as \(2^4 = 16\) covers the requirement.

DHCP Options Precedence

The order of precedence for DHCP options is Reservation > Scope > Server > Class.

Matching DHCP Option Codes

• 003: Router

• 006: DNS Servers

• 015: DNS domain name

• 007: Not a DHCP option, an MI6 spy

DHCP Renewal

• A PC will first attempt to renew its IP address when 50% of the lease time has expired.

• If the first attempt to renew an IP address fails, a PC will have a second attempt when 87.5% of the lease time has expired.

• If the PC has not successfully renewed its IP address, when 100% of the lease time has expired it will send a DHCPdiscover broadcast.

DHCP Packet Order

• DHCPDISCOVER

• DHCPOFFER

• DHCPREQUEST

• DHCPACK

DHCP Negotiation

• As a unicast, a DHCPoffer packet

• As a unicast, a DHCPack packet

• As a broadcast, a DHCPdiscover packet