The Directory Server package refuses to install on unsupported models - so the first step of any Synology domain project is a two-minute compatibility check, not a purchase.
Synology Directory Server is free, but it is not available for every NAS. Synology publishes an applied-models list for the package, and Package Center simply will not offer it on models that are not listed. This page explains which families qualify, why the small models are excluded, and how much memory the domain controller job honestly needs.
Directory Server is a heavier service than file sharing: it runs a Samba Active Directory DC with Kerberos, LDAP and DNS integration. Synology limits it to models with enough CPU and memory headroom. In practice that means the Plus series and everything above it (XS, XS+, and rackmount models aimed at business use). If Package Center does not show Directory Server on your NAS, the model is not on the list - there is no sideload or workaround worth trusting for a domain controller.
Entry J-series and basic value models are built for one job: storing and serving files on a tight budget. Their processors and small fixed memory are fine for that, but a DC must answer every login, policy refresh and DNS lookup for the whole office, all day. Synology's line is reasonable: a controller that stalls at 9 AM when everyone logs in is worse than no controller. If your NAS is a J model, the honest options are a Plus-series NAS for the domain (it can also take over file duty) or keeping the J box as pure file storage beside a small Windows or Linux server.
Directory Server itself is not a memory monster - a domain for a few dozen users and computers is a small database. The real pressure is everything else on the same box: DSM itself, file services, and any other packages you run. As a working guide: 2 GB is workable for a tiny office doing DC + light file sharing, 4 GB is the comfortable point most Plus models ship with or accept as an upgrade, and businesses running Surveillance Station, virtual machines or many packages should separate roles anyway. A slow DC usually means the NAS is overloaded with other jobs, not that the domain is too big.
If you are purchasing with a domain in mind, buy a current Plus-series model with upgradeable RAM and keep the DC's workload clean: files plus directory service, not a playground for every package in the store. Two bays are enough for most small offices with mirrored drives; the controller's database is small - it is the files that decide your disk size, not the domain.
The video's NAS runs the whole domain demo - watch which model class it uses and how Package Center offers the Directory Server package.
Watch Full VideoAlmost certainly your model is not on Synology's applied-models list for the package. This is common on J-series and basic value models. Confirm on the official specifications page for Synology Directory Server - if the model is absent, the package cannot be installed on it.
Yes, several supported Plus models use ARM processors. What matters is being on the official model list, not the CPU brand - plus enough RAM headroom for DSM, file services and the directory together.
Up to a point. Logins are light work; where a stronger model helps is when the same NAS also serves many files and packages. A dedicated, lightly loaded Plus model feels instant for a small office domain.