DHCP Server configuration

  • INTRODUCTION

Dynamic Host Configuration Protocol (DHCP) is the protocol used to allocate IP-addresses automatically to computers, networked equipment (printers, disk drives) and handheld equipment. This is the system that enables your computer to obtain information automatically from routers or domain servers.
DHCP can be used to configure IP-addresses, DNS servers, gateways, domain name and more.

  • NOMENCLATURE

By DHCP client we mean anything that has its configuration done by the DHCP server. This might be a printer, a disk drive, a computer or even a cell-phone with Wi-Fi. Most often, a DHCP client is simply a computer onboard the vessel.

The Lease time is measured in seconds, and is used by the clients as an indicator for how long they are allowed to stick to their IP address and configuration. In theory, a client can ask for configuration from the DHCP server, get it – and not talk to the server again until the lease time has passed. Most commonly though, the client will “check in” every now and then to make sure it has the correct configuration. The lease time is the time in seconds a client is allowed to hold on to its configuration.

  • DHCP SERVER CONFIGURATION

To be able to start the DHCP server, the DHCP server field, range from and to, subnet mask and lease time MUST be configured. In addition, a gateway should be configured in the option table, as well as a DNS server. Without the gateway and the DNS server, there will be no internet access for the clients. The DHCP server will not be activated unless the activate checker is checked, and all the required fields are configured.

dhcpserver.JPG

DHCP SERVER: The server IP-address is the address of the network card connected to the network being configured by the DHCP server. This address MUST be configured manually on the computer running the DHCP server. Note that if this address is received using automatic configuration – you probably don’t want to set up another DHCP server in the same net.

SUBNET MASK: This is the subnet mask for the network. It should be the same as the DHCP server machine is set to.

RANGE START/END: This is the range from which all the computers in the network will get their address. Common examples are 192.168.1.1 to 192.168.1.100 and 10.0.0.1 to 10.0.0.100. The maximum size of a range is 255 computers.

Both examples above leave room for 100 computers simultaneously. Note that addresses can be allocated statically to computers outside of the range – in fact, it’s a good habit that static addresses SHOULD be placed outside of the range so they won’t get confused with the addresses being dynamically allocated. This also assures that an address being allocated statically from ashore won’t get allocated to another client in the replication time period.
Make sure that the network range matches the address of the DHCP server. If the DHCP server has address 10.0.0.1, the range must be configured in such way that the clients receiving configuration from the server can reach it. (I.e. 10.0.0.2 to 10.0.0.200).

LEASE TIME: The lease time is the number of seconds a client is allowed to stick to the IP-address they are given (the lease) by the DHCP server. Most clients will start asking for an extension after half the lease has passed. As the DHCP server won’t tell the clients to release – the lease time greatly impacts the time it takes to change network settings. A short lease time allows for fast changes, while a longer lease time lets the computers hold on to their IP address for a longer time.

Examples:

28800 seconds  8 Hours
86400 seconds  24 Hours (recommended)
604800 seconds  1 Week


The maximum number of seconds allowed is 4294967294 seconds. This is interpreted by most clients as infinite. This should only be used after serious consideration, as network changes would now require a manual release from the computer (and manual is just that – it has to be typed into the computer).

  • DHCP OPTIONS
Router This is the gateway address that the DHCP client will use. The value is an IP-address to the gateway. If the DHCP server is running on the same computer as DuaCore Pro, this should be set to the same as DHCP server IP.
This field can be applied more than once, but it is not recommended as DHCP clients can choose whichever router they can.
Time Server The values must be an IP address, and it should point to a NTP time server.
DNS Server This is the name server that will be used by the DHCP client. The value must be an IP-address, and it can be inserted as many times as needed.
Domain Name This is the DNS suffix that will be applied to this connection. The value is a string, and can only be applied once. This is useful only if the clients have fully qualified domain names on the vessel.
MTU Interface The value must be a number, and it defines the MTU of the DHCP client’s interface. This can only be applied once.
Same MTU in subnet The value must be either 0 or 1. 0 means that the DHCP client should assume that some networks directly connected to the DHCP client’s network might have smaller MTU. 1 means that all subnets share the same MTU. This can only be applied once.
Proxy autoconfig To be able to let Internet Explorer automatically detect the Dualog Web4Sea proxy in the network, this entry can be used. This is a string, and it should typically be configured to:
http://<IP-of-Connection-Suite>:3000/proxyautoconfig
This entry should only be inserted once. It is possible to add more, but it will confuse the Internet Browser.


 

 

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