Subnet Addressing

The configuration of the Subnet define how Machines receive network settings (IP addresses, routes, DNS, NTP etc.).

There are two adressing modes:

Addressing	Who hands out network addresses & settings	When to use
DHCP	A DHCP server provided by the customer	If there is already an existing DHCP server that should/must be used
IPAM	Foundry itself	If there's no DHCP server or if the network configuration should live in meltcloud

DHCP

In a Subnet with addressing mode DHCP, a DHCP server (configured by the customer) must provide the following settings via DHCP.

Missing or additional settings can be specified in the Subnet's configuration in meltcloud.

Setting	Used for	Example	DHCP option	Required	Can be specified in meltcloud
IP address	The Machine's address on the segment	10.0.10.11/24	the lease itself	yes	no
Gateway	The default route	10.0.10.1	3	yes	no
DNS	Name resolution	10.0.10.53, 10.0.10.54	6	yes	yes
NTP	Time synchronisation	10.0.10.60, 10.0.10.61	42	no	yes
Search Domains	Resolving unqualified names	lab.example.com, example.com	15 and 119	no	yes
MTU	Frame size	9000	26	no	yes
Routes	Reaching other segments	10.0.20.0/24 via 10.0.10.1	121	no	yes

If both the DHCP server and the Subnet configuration specify a value, the Subnet's value takes precedence.

Example Kea configuration

{
  "Dhcp4": {
    "subnet4": [
      {
        "subnet": "10.10.10.0/24",
        "pools": [
          { "pool": "10.10.10.100 - 10.10.10.200" }
        ],
        "option-data": [
          { "name": "routers", "data": "10.10.10.1" }, // sets default gw / route
          { "name": "domain-name", "data": "lab.example.com" }, // dns search domain
          { "name": "domain-name-servers", "data": "10.10.10.53, 10.10.10.54" }, // dns servers
          { "name": "ntp-servers", "data": "10.10.10.60" }, //  ntp source
        ]
      }
    ]
  }
}

As Terraform: DHCP Subnets.

IPAM

In this case, Foundry owns all IP addresses and network settings, no DHCP server needed at all.

The settings are specified in the Subnet itself:

Setting	Used for	Example	Required
IP Pool	An IP Pool provides IP addresses	10.0.10.0/24 from IP Pool wl-prd	yes
Gateway	The default route	10.0.10.1	yes
DNS	Name resolution	10.0.10.53, 10.0.10.54	yes
NTP	Time synchronisation	10.0.10.60, 10.0.10.61	no
Search Domains	Resolving unqualified names	lab.example.com, example.com	no
MTU	Frame size	9000	no
Routes	Reaching other segments	10.0.20.0/24 via 10.0.10.1	no

As Terraform: IPAM Subnets.

Pinned/Static IP addresses

By default, IP addresses that are distributed from IP Pools to Machines are randomly assigned and not guaranteed to be static over time.

In general, unstable Machine IPs are sufficient: Kubernetes workers do not require static IP addresses or DNS names. However, if there is some external system (or legacy guideline) that expects to reach Machines via static IPs (or DNS Names that point to static IPs), you can pin IP addresses to Machines.

Enrollment on IPAM Subnets

If a Machine enrolls on a Subnet with IPAM addressing mode, it tries to allocate an IP address from the IP Pool by using RFC 5227 (ARP probing).

There might be some network setups where this approach doesn't work - in that case, check Enrollment fails on an IPAM Subnet.

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Next, see how Network Profiles attach a Machine to these Subnets.