Technical Tip : DHCP option codes

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Description	This article provides a list of commonly used DHCP option codes along with descriptions.
Scope	FortiGate, DHCP.
Solution	A DHCP (Dynamic Host Configuration Protocol) option code is a numeric identifier used in DHCP messages to convey specific configuration parameters from a DHCP server to a client. These parameters can include things like IP addresses, subnet masks, gateway addresses, DNS server information, and more. DHCP option codes are essential because they allow for the dynamic and automatic configuration of network settings on client devices, reducing the administrative burden of manually configuring each device. By providing standardized codes for various network parameters, DHCP ensures that devices receive the correct information for seamless network operation, making it a crucial component in simplifying and efficiently managing IP address allocation and network configuration in a dynamic and rapidly changing network environment.  List of commonly used DHCP option codes along with their descriptions: Subnet Mask (Option 1): Description: Specifies the subnet mask for the client's network. This option is essential for defining the client's IP address within the correct subnet. Time Offset (Option 2): Description: Provides the time offset from UTC (Coordinated Universal Time) in seconds. It helps clients adjust their clocks for accurate timekeeping. Router (Option 3): Description: Lists the IP addresses of the routers or gateways on the network. Clients use this information to determine their default gateway. DNS Servers (Option 6): Description: Specifies the IP addresses of DNS (Domain Name System) servers that the client should use for name resolution. Helps clients resolve domain names to IP addresses. Hostname (Option 12): Description: Provides the client with its hostname. The client can use this information for identifying itself on the network. Domain Name (Option 15): Description: Specifies the domain name to be used by the client. It helps clients in constructing fully qualified domain names (FQDNs). Broadcast Address (Option 28): Description: Indicates the broadcast address for the client's subnet. Used for broadcasting messages within a specific subnet. NTP Servers (Option 42): Description: Lists the IP addresses of Network Time Protocol (NTP) servers for time synchronization. Ensures accurate timekeeping on client devices. NetBIOS Name Servers (Option 44): Description: Specifies the IP addresses of NetBIOS name servers. Used for NetBIOS name resolution on Windows networks. NetBIOS Node Type (Option 46): Description: Sets the NetBIOS node type for NetBIOS name resolution. Controls the method used for resolving NetBIOS names. Vendor-Specific Information (Option 43): Description: Carries vendor-specific configuration information, typically used by specific vendors or devices. Allows customization of DHCP options based on vendor requirements. Classless Static Route (Option 121): Description: Provides routing information for subnets within a larger network. Useful for directing traffic to specific subnets through appropriate gateways. MSFT-IE Proxy Server (Option 252): Description: Used by Microsoft clients to specify proxy server settings. Configures proxy server information for web access. Option Overload (Option 52): Description: Indicates whether options are stored in the sname or file fields of the DHCP packet. Used when there are too many DHCP options to fit in the standard fields. Client Identifier (Option 61): Description: Identifies the client within the DHCP server's database. Useful when multiple clients have the same MAC address. TFTP Server Name (Option 66): Description: Specifies the name of the Trivial File Transfer Protocol (TFTP) server for booting. Commonly used in network boot configurations. Bootfile Name (Option 67): Description: Provides the name of the boot file for booting via TFTP. Specifies the file to be downloaded by the client during network boot. Lease Time (Option 51): Description: The Lease Time option specifies the duration for which a DHCP client is allowed to use the assigned IP address before it must renew the lease. It is expressed in seconds. When the lease time expires, the client must contact the DHCP server to renew the lease or request a new IP address. This option helps manage IP address allocation and ensures that IP addresses are not held indefinitely by inactive clients, allowing for efficient IP address reuse within the DHCP scope. These DHCP option codes are essential for configuring and customizing network parameters for DHCP clients, ensuring proper network operation and connectivity.